I would recommend you to try as many guns as you can and make a decidion based on what you feel is most comfortable.
Each person will have their own opinion, mostly swayed by whatever gun they are shooting or sponsored to shoot.
If you are serious then i would say that if you buy a crap gun, you will end up buying another in a few onths when you realise it isnt that good. Best bet is to by right first time round. many guns are easily upgradeable but here are a few to try:
Impulse: Smart Parts gun, they often send out guns that end up straight back in the shop for fixing, as they do not properly field test their guns. Once set up right, they are reasonably reliable and a solid investment. They usually retail for around £300 second hand on these boards.
www.smartpartseurope.com
Cockers: Planet make decent enough guns. The Eblade are reliable, and their customer service is second to none. Any problems they will sort out for you. The cocker is the most easily upgradeable gun, and you can pay anything from about £300-£1300 for one depending on how good you want it to be. The Eblade is a choice of many teams in the UK because planet are easy to get hold of if you have a problem. They are excellent starter guns, but in my opinionm are not fast enough to compete at a high level, but as you are just starting out, should be ideal.
www.planet-eclipse.com
Intimidators and Matrixes are awesome guns, fast, light, and available in many styles and shapes. Something for everyone. Having said that there are very few people in the UK who can tech them, and are also quite expensive. The choice of many of the top teams in the world though, which counts for something. I have an Alias and think that it is the best gun i have ever owned, having previously had a cocker and an Angel LCD. The reason i have an alias is because i wanted something small and light, that was insanely fast. Thats what i have got, accuracy wise it is ball on ball with the right barrel. Price ranges from £500 - £1400 depending how old and used they are
Angels: UK based WDP. Paintball innovators, they come out with guns more frequently than Nick and Robbo have arguments
Often set up very well, they are reliable and insanely fast. WDP have excellent customer service on a par with planet. Something for everyone here, as they have many types of gun, with varying price ranges
www.wdp.tv
As for a loader, a Halo is a popular choice, as is a viewloader Egg. Viewloader Revvys are good as well.
Air wise, presets are the way forward. easy to use and maintain and easy to get hold of. Try PMI europe or the p8ntballer shop for one.
All i would say is ask someone if you can try their gun at an event or something. Players will always let you, and are always keen to talk about their guns. Be wary of people pushing their gun on you too hard though, i.e dont ask someone from Nexus what gun you should get as they will lean towarsd the eblade, or someone from the Kelly's as they will be loyal to WDP who sponsor them.
Best thing is to listen to players from lower teams, who have the guns they have not because they were free, but because they actually like them. Ignore people who try to claim that one gun is more accurate than another gun, its crap. As long as you have a decent barrel, and decent paint, then every gun will be as accurate as any other gun.
Goold luck and i hope that helps.