My two cents
That's right, American currency. Sorry, but that's all I have.
accuracy
As pointed out above, paint quality is gonna be the most important single factor there. Bad paint breaks in the gun more, bounces off the other guy more, and hooks more than good quality stuff. Spending another 15 bucks (sorry about the currency once again) on superior paint is well worth it in my view.
The other things that will have some effect on accuracy include consistent velocity, recoil, and the likelihood of breaking paint. Consistent velocity means that your arc will be more predictable, in other words, vertical aiming is more possible. If one ball is zipping along at 40 fps faster than the last, it will hit far higher than the last ball. If you are shooting without pausing to aim between shoots, but instead trying to correct from previous shots, shooting as fast as you can pull the trigger, the recoil between shots can make the aim less predictable. Blowbacks kick like mules. That would include Spyders and Tippmanns. This is one place where the ION has a real edge over those two, since it is a simple spooler. Lastly, breaking paint is going to cause balls to go every which way until the inside of the barrel is evenly coated throughout (which ain't quite ideal) or until the barrel is cleaned. Looking for a gun with some sort of reliable anti-chop is important if you want to hit what you are shooting at.
hierarchy of markers
If you know that you are planning to compete in the next year or so, there is no reason not to get the best marker you can afford. It is not important to own every marker along the scale from Talon to DM6. There are no cards to punch. But that only applies if you are going to compete within the next year or maybe two on a frequent basis.
If you are only wanting your own gun to address some specific deficiency in the rental guns, you should shop on that basis. Check to make sure that at your site there are no restrictions on any gun you might be thinking of buying. If no electros are permitted, then if want an ION you have to find a new site, if your site doesn't fill HPA, then you won't want to buy a gun like an Angel that only breathes HPA. If you want to shoot a ramping gun, you should make sure that it is legal for you to do so at the site you plan to attend or else find another place to play where it is legal.
If you plan to maybe play a tournament or two if the opportunity arises, don't spend a ton of money on a tournament ready machine gun. That would just be silly. You don't need to buy a 3 ton sport utility vehicle to cruise downtown to pick up a few things at the store, unless by cruise downtown, you mean through the DMZ, by store you mean ammunition cache, and by few things, you mean mortar shells and reinforcements. Similarly, spending 400 GBP on a gun is excessive if you are playing reincarnation in the woods with your buddies, and don't know or care who Ryan Greenspan is.
It makes more sense to spend a little less on, say, an autococker (assuming that there is someone around who can fix them, or assuming that you can use the internet and tools) which will burn through less paint, serve you well for years, and give you a legitimately better gun performance than a blowback without causing you to shoot a ton more paint before you decide to give up your financial well being for an obscure sport with a low rate of return on investment and high degree of hostility and frustration- not that I'd try to scare you off. Blowbacks are ultimately blowbacks, and blowbacks that shoot faster are ultimate blowbacks that chop paint in an even bigger hurry while bucking around so much that random chance is the only reason they do more damage on the field. If all the bang-bang noises are your thing, and high paint bills don't dissuade you, then that (an electro-blowback like a e-Tippmen or e-Spyder) is the way to go.
smart money
There are some things that you will need no matter what gun you have. There are some items that are useful in almost all cases. This is how you avoid wasting money.
1. Buy a nice, comfy mask. It should not fog. It should not chaff your baby soft skin. It should not leave exposed your gaping jaw or pinch your ears or temples. If you shake your head vigorously, you should get a little dizzy long before it comes off your head or becomes misaligned with your face. I like the Profiler. I say this because the lens is easy to change, and I am a little princess when it comes to clean, unscratched lenses, and because it stays put while covering my head adequately. There are other good masks out there, and if I had a different head, I might prefer a different mask. This is something that must be tried on to get it right. Any mask you buy that is at the top of the range offered by a company will probably be pretty good, but to find your best fit, you should try out some different ones.
2. If your site fills HPA, get yourself a 68 ci 4500 psi tank. I am not even kidding. This is the most commonly used tank, middle of the range for volume, with the higher of the two common fill pressure ratings (3000 psi and 4500 psi). This is the tank you will wish you'd bought in the first place if you play paintball for long enough. Get a preset screw in tank. You can use it in any application where you'd use a CO2 tank, but your velocity will be that much more consistent, you will have that much less trouble with liquid working its way into the gun, you will replace fewer o-rings, and if combined with regular flossing, you will have fewer cavities. This will work with almost any gun. When you get a new gun, you will use the same tank. Let me repeat that you should make sure you can get it filled, then go get one if you plan to play more than say 20 times.
3. Look for guns with 'cocker barrel thread. I don't get why everyone doesn't just on the bandwagon with this, but most good guns use the barrel thread style used originally in the WGP Autococker. There are a few weird ones out there, and I suppose they have their reasons, but it is annoying that this is not the standard, since it is the defacto standard.
The reason you want to try to score a gun with cocker threaded barrels is that no matter where you go from there, you will be able to take all your barrels with you. If you start out with a WGP Trilogy, you can get cocker barrels. Say you get an old Eclipse Director's Cut Edition Autococker a couple years down the road. You can still use all your barrels. Then a year later, you sell that and buy an Intimidator. Same barrels are still useful. Then you decided to try out a DM4. Guess what? Barrels are still all good. Then you think, maybe a Shocker would be a good buy. If you don't get one of the private label Shockers, you will get a gun with a new type barrel thread. Now all your barrels are in a holding pattern until you buy another gun with cocker barrel thread. No problem; at this point you are trading guns out every six months. But my advice is still to shop for something that will be compatible with most of what you are likely to own in the future.
A second choice would be something that uses Spyder thread, since there are quite a few of those around. All the same, it is a distant second choice.
conclusion
What was I talking about again? Oh, crap, I accidentally wrote a whole PGI article just to answer a simple question. Actually, this about 200 words longer than the last article I wrote, I bet.
Decide what kind of paintball you are interested in and committed to playing before spending any money. Then don't spend money on stuff that you don't need and will outgrow in a few months. Then plan to spend most of your money actually playing paintball instead of obsessing about your stupid gear. Everyone else will tell you your gun sucks anyway, so go learn how to shoot them so that you don't start thinking they know something you don't.