In 250's you can use any chassis (a few of the guys have made their own) but only a selection of engines, most of us use the honda cr250, but we have Rotax 257's, KTM 250's Yamaha 250's etc....I'll try to remember to bring a copy of my blue and gold books down for you tomorrow, they have all the details and rules of karting in them. Other classes have a selection of chassis to choose from but they have to be homologated, so no making your own

, and they have specific engines for each class, most of which are sealed which means you have to send away for any maintenance work or rebuilds.
I picked up my first gearbox kart for £200, so there are decent bargains out there, I would definitely try and get advice on any kart you look at before you buy but most people in karting are fairly straight up (in my experience).
I dont know about twin engined karts, there arent any running at Dunks.
Stu