At £45 though I will probably get it!
Find out if a site local to you does 'walk-on' days for 'own gunners' first.
If you are on limited funds I'd suggest something unbustable, like a Tippman 98 or BT 4, you'll obviously need a decent set of goggles and a paint hopper for your gun, something like the Proto Primo is cost effective and maintenance free whilst improved over the stock gravity hoppers. You'll need to find out if your local site runs CO2 or compressed air and if they will let you use your own tank (if so air is preferable and you can pick up a 48 cu in 3000 psi system quite cheaply). Then your biggest on going expense will be paint!
This set-up will serve you well until you have the funds to upgrade. Many people play for years with kit like this.
On the Mongoose front, its cheap for a reason, and will be hard to get parts for, keep serviced and set up. Same for Piranhas etc. I'm pretty sure Apocalypse do offer walk-ons. If you need any more help on the equipment side, check out LiPS Paintball's getting started guide:
http://lips-paintball.com/acatalog/Getting_Started.html or get down to Welwyn and visit them, or call 01707 325007
Hope that helps.
P.S. that £45 will probably be the worst you could ever spend it on and you'll never get it back!
