STick wit the A-series all the way. I remember reading a few articles on the K-series conversion a few years back and it seems to be a lot more bother than it's worth. THe ground clearance at te front to the engine is also a nightmare when the K-series is in a mini shell - Seen a few at various shows...
If you *REALLY* want to convert it, the Vauxhall 16v twin cam or Honda Vtec are the only ways to go!
There's a company out there that specifically buids subframes to mount these into Minis, though they're not cheap! (roundabout £3k if it remember rightly... though I could be wrong )
If you choose to convert, be aware that whatever one you choose, there'sll be a fair about of bodywork extension and chopping, in the case of the Vauhall and Honda engines you'll need to add at least 3 inches to the length of the front of the car so the engine will fit! Unless you fit a Clubman front end.
I've only ever seen one round nosed mini where this extension actually looked professional enough not to look out of place and wrong....
And that was the guy up in Aberdeen (I think his name was Leigh?) and it turned out like this: