As for the topic in this thread, there is a lot of information out there. I think the easiest way to go has been suggested and that's to buy and build up a 951 engine and do the transplant. You can also adapt a 968 'box to fit and again this draws differing opinions but there are plenty out there who think the 6 speed is stronger than the 5 speed. A lot of high hp racing 951's have a 968 box in there. If you're only shooting for 350-400whp (and this is a very quick car!), you will be fine with most of the stock driveline components. Also as discussed by me and Flash, you will or should really budget for suspension and brake upgrades too. If you want more than 350-400whp you can still do this but might need to enlarge the 2.5L out to 2.7L, 2.8L, or 3.0L+ by a variety of ways. I am doing a 3.0L 8v as we speak (pistons went in today) that is from a 2.5L and dry sleeved out. It will have a number of upgraded components and be approx 500hp at the crank we figure. The advantage of the turbo (2.5L) block is the bespoke turbo mount/inlet etc. I also have a larger 16v motor being built too, but that will have many custom components on it and is not really a good example for this topic.
I know Mike with the 3.0S2 cabriolet that you've all been mentioning. His car was very well done, but it went pop and now he's shooting for 500-600hp at the crank. Also Zoran's car has been through many metamorphoses and had LOTS of money spent on it.
I have another friend down here who is doing his 968 and that will have the turbo on the exhaust side way up front (this is going to need lots of R&D!) so there are lots of us down under continuing to play around with these front engined water pumpers. I think if you absolutely have to have a turbo'd car you should stick to the 951. Even so there are ways to do it and ways not to. Unless you really have a great deal of money to blow on this, you will most likely run into issues.
The only other guy that I would read up on is Markus on Rennlist who did a great job with his Orange 'Jaegermeister' 968. Well worth reading up on.