I'm about £3500 into a rebuild and vauxhall small block conversion on my mini and I built my own frame.
Realistically you are going to spend around £250 on a donor car (buy a complete donor, it makes life a lot easier), the best part of a grand on a conversion kit when you include drive shafts and the modified lower arms, £100 or so on brakes once you have bought them and rebuilt them, another £350-500 on an after market ECU if you need it (and a lot of them do), £50-100 or so on a manifold/throttle bodies (most standard manifolds don't fit so have to be modified or you have to cut the bulkhead), then you have the exhaust to sort out and the clutch and brake master cylinder to deal with. A pedal box will set you back another £300 or so.
You will also need to be able to weld as none of the conversion kits offer the gear linkage already sorted out. You will have to cut and modify the existing one and possibly fit a bulkhead box to clear this as well.
Then add on about another £500 for all the other bits and pieces you will forget like pipe work, radiator, speedo cable, fuel tank, fuel lines and fittings, hilos to corner weight it properly, tracking check after its been put together, etc, etc.
A 1275 a series isn't sounding too bad now is it?