Back on topic -
1st things 1st - If you're wanting to get into tourney speedball - I dunno how much experience you have but I'm gonna start mega basics here. I'm guessing the Ion will be your first electropneumatic marker right? If i'm teaching you to suck eggs here, sorry - but I know when I got my 1st electro I wasn't sure on all this stuff...
Number 1 priority is really running off Air not CO2. Air will not blow the seals on your marker like Co2 because it doesn't cool as it expands. Its also far more consistant and will allow you to shoot faster than Co2.
Number 2: Fast Hopper. The Ion will shoot 17 Balls per second in semi if you can pull the trigger that fast, or consistantly in ramp (or Rebound as Smart Parts call it), but only if you can ge the paint in there fast enough. The eyes will stop the ion firing till the paints there. A revy hopper like a Veiwloader revolution will feed upto about 8Balls a second. The Egg hoppers will just about keep up but you might have some gaps in the feed. In reality a Halo B or Empire Reloader B is the way to go. These physically force the paint into the Marker, faster than gravity.they will feed upto 23bps consitantly, more with upgrades.
Number 3 (really equal with 2) is that feedneck. The clamping Smartparts Qlock is the most popular and will hold any hopper tight on top, because its adjustable.
Only when you have these would I start to worry about the stuff everyone else is talking about. The barrel could really do with replacing but that could be said for almost any marker's stock barrel. I'd recomend a freak kit as you can switch inserts to match whatever paint you are shooting on any given day. This improves accuracy and efficiency on air. - The Ion takes Impulse threaded Barrels.
The ASA (Air source Adapter) is very basic on the Ion. I would recomend any on-off adapter, I use a planet eclipse one wich works very nicely. I would avoid drop forwards, You don't see any pros use them nowadays, and lets be honest, they know what they're doing;-)
Trigger is personal preferance again but I still use the stock one and it does me fine. You can adjust it plenty mine has a pull of about 1mm and stops as soon as it fires the marker so you litterally just tickle it. It rips
After that - Its up to you and depends how far you want to take it, but upping your Ion will become addictive. Give Jimmy at Warped Ion Centre in Wolverhampton a call, He stocks basically everything you can get for Ions and shoots one himself. Best of all he won't sell you anything if you don't need it.
www.warpedpaintball.co.uk
All the best, PM me if I can help any more
Scooby