I'm not familiar with the Rover lumps, but I thought the casings are the same for all the 'boxes used with the k-series, it's just the ratios that differ?
A honda K-series will cost you ££££ - physically fitting one in the car is easy, but the engine and parts to make it work, are excessively pricey:
Engine / box with OEM Ecu - probably around £1500 (best case - they vary a lot)
Driveshafts - £200 a side, if you get bespoke ones made, or slightly less if you cut and shut civic shafts on to rover shafts.
ECU modifcation - £700 (unless you go for marlin's £300 "box of tricks").
Fuel pressure regulator £50
Clutch kit £200
and many more odds and ends that I've forgotten....
..... have I put you off yet?
Higher output k-series has to be the way forward for simplicity / cost reasons?
Having said the above, and just to spice things up a bit: I almost put a VAG 20vt lump in my car, having played around with them quite a bit, and having good contacts for mapping / parts.... Engines have been going for buttons on ebay recently (a friend got one for £250 the other week), and for that sort of money you can't really go far wrong? If you get one with a k03s turbo, then the lag is negligible, and a very reliable 230bhp is easily obtained. They've been used to good effect in Elises!