They are quite good on petrol as are most 4 cylinder turbo charged cars as long as you can control your right foot. The ST170 is known to be quite poor in that area.

You could probably do a really basic conversion or an RS conversion for £3-3.5k if you did the work yourself, it's an expensive business.

ESP is electronic stability control and almost all the early 2.0 Focuses came with it, Ford named them ESP's although there are Ghia's too but they are 5 doors.

If you only have £2k then I'd try and find the best 2.0 you can, most ST170's will be a bit dodgy or high mileage at that price. You could also find a diesel but you might struggle to find a 3 door.