You can't tell what sex they are until they are a few years old, usually by the age or 3 or 4 it's pretty obvious but earlier than that would just be guessing with most species.
Adult females of all species are bigger than the males when fully grown, like torts the females have shorter, thinner tails and the cloaca is closer to the base of the tail, males have longer, thicker tails and the cloaca is nearer the tip. This difference in the tails is more pronounced in some species than others, in Maps for instance it's very obvious and from quite an early age.
Males of most species also develop longer front claws, male sliders in particular have much longer claws than the females, up to an inch long for a fully grown male and there's no mistaking them, in other species it may be less obvious.
Cheap tanks are available from many sources, you'll often find that the bigger the tank the harder it is to sell and therefore cheaper, but you have to be able to collect it of course! I've seen loads of perfectly good 8' tanks sold really cheap or even given away, but you need a van or trailer and a team of strong guys at each end of the journey.
Try Freecycle, I've had a few good tanks from there, for free of course. Some nice big tanks come up on eBay, and something like a 4 x 2 x 2, or 6 x 2 x 2 usually fetches well under £100, I saw a 4 footer go for just £16 last week. And the fish section of Pets Classifieds is another good place to look.