You can get a manifold that allows the turbo to clear an un-modified bulkhead. As for the rest yes it is an easy conversion. The engine and box are the same as a standard Mini one anyway so as long as his current engine is in good condition it probably wouldn't even have to come out. The Metro turbo front brakes will even fit a Mini with just four special bolts to hold the calipers onto the hubs (Rover did it on the ERA's). Costs are pretty minimal and I know people who have managed to do it for under £200 by sourcing the parts from wherever they could lay their hands on them.

Different engines require different subframes and can mess with the suspension geometry as others have said and the costs skyrocket if you buy the subframes off the shelf.

You can also fit a BMW K100 motorbike head to an A-Series lump and see impressive gains. Cost-wise this is going to depend on how much of the work you can do yourself. You need to do some machining to the block and head but not much. You then need to sort either throttle-bodies and the associated injection gubbins or a suitable set of carbs. I'm doing this slowly to a lump that's destined for my GTM.

Iain