look on the classifieds and contact some of the sellers, meet up at a tournament and get to have a go. if you like it, go for it.
You'll never get a definitive "buy this marker it's great" answer on here, as we all have our own opinions. I don't think many will argue that for a new starter, a second hand marker from a tourney player who's trading up is hard to beat. As for type, a second hand shocker/cocker/impulse etc.. will out-perform a new marker at the same price, so go for a second hand marker on CO2 now, about £200 then get a decent hopper then a freak set or similar when your set, then trade up to air, then start adding the extras. I know a couple of players who's markers have kept improving as they have. If it turns out that you hate paintball, then sell it on, you'll have more look shifting a quality second hand marker for anything near what you paid, than trying to sell a marker you bought brand new. A second hand cocker at £200 this week is still worth that next week, a brand new spyder today is worth a helluva lot less next week. (Not criticising the spyder, it'd be the same for any marker).