If you want to end up with an expensive A2 petrol (or diesel come to that), start off with a really cheap one then get a garage to sort it out for you.
Honestly, buy the best you can afford and keep it nice. The initial price difference (say going from £400 to £1400) will barely buy you a better engine, the inevitable clutch / fluids / service items and perhaps 15 hours of labour, let alone the myriad of other things a tired one will need. You still end up with what was a cheap A2, it's now just a cheap A2 with a different engine that owes you an extra £1000 +
Now ... if you were a DIY mechanic that could tackle all the work, then game on. Even then it's a moment's work to drop £400-800 into one of these and that's before you consider an iffy engine.
e.g. my current TDi came in to me at £300. It had been fairly well looked after, good mechanicals, recent tyres all round, good body and so on. Even so, with me DIY'ing and being sensible (not gone mad) I've gone past £900 all in. There's no one big expense, just a lot of bills for £10 to £150. Those £20's, 30's and 40's really add up, let alone when a garage is burning through £40-65 per hour, possibly plus VAT.
Sorry, that all sounds negative: the 1.4i is an absolutely cracking car so well worth hunting down, but if you're not DIY'ing, try to find a really nice one. If you're not sure, see if someone on here can assess it for you before you commit. At the very least, post any queries on here: someone will have a good / sensible answer for you.
Good luck!