I would put this query on the TortoiseTrust site if I was you and seriously think about rehoming from them rather than buying too, as there are so many which need good homes out there. The sooner it becomes illegal to sell these poor little things in petshops IMO the better.
I'd start with a big tank... sooner or later these guys are going to grow so starting with a 3 or even 4ft tank means less money in the long run!
You'll also be wanting a water heater, basking lamp, UV bulb, somewhere for them to bask, a filter, a fair amount of kit overall so I'd have a real good look into the different breeds & what their specific requirements are.
I've heard stinkpots (musk turtles) can be good for beginners although I started with a yellow belly. Steer clear of soft shells though.. although they are really lovely as babies they're tricky to look after and can grow real big!
Look around for care sheets and web sites like proteus reptile trust for useful info.
Turtles are just the best thing ever but once you have them the research never stops!