I can speak to converting a 944S - my brother did the same thing to an '87 944S several years back. I will ask him for specifics (if there are any) on what he had to do for wiring for the gauges and whatnot.
All in all, the 944S and regular 944 are almost identical. I would consider it a very worthwhile way to go about building a car. I, also, didn't have the $$ all at once to buy a ready-made car - though they can be quite worth the money.
If the car has ABS, the removal is easy - the only part that will require some head-scratching is the jumping out of the lights on the dash. I would highly recommend doing it the "right" way so you can use the Master Caution like it was mean to be used.
The gear ratios for a 944S transmission are slightly taller for 1-4th gears, but slightly shorter in 5th gear.
Suspension (to my knowledge) is all the same - you'll be doing shocks, springs, tortions, etc with a new build, so don't worry about the condition of that stuff. A-Arms are *definitely* the same, so no worries there. Steering is exactly the same. The entire body is exactly the same. I like the "S" as a base car because it keeps you with late suspension geometry and electronics - my personal preference - to each his own, though.
Some questions I would have when considering the car are:
- what is the engine plan? (where to get one, how much, etc.)
- is the engine in the "S" worth selling? (to my knowledge, there are a few parts that cross-over, but not that many. I would plan on parting out the engine or selling it outright)
- what's the overall condition of the car and what parts are you planning on reusing?
- **DOES IT HAVE LIMITED SLIP?? This would be a huge +++
I hope this is useful for you. I can tell you it works - I've seen it done firsthand.