I have a 944S2, and just got a new head this fall, due to a crack that developed in the old head.
I've owned the car since 2001, and bought it with 115,000 on the clock. It now has about 150,000 miles. I've had a lot of work done to the car - mostly preventitive maintenance, but some 'unexpected' repairs, like the head.
The primary use for my car is DE and autox, so needless to say, I drive it pretty hard. Driving any car hard will increase the maintenance costs - that's the nature of the beast! That said, I do not know of any 150,000 mile car that could be subjected to the amount of stress I put on my 944S2 that wouldn't cost MORE in upkeep than a Porsche. For example: even when running with a cracked head at DE's, the car would still pull strong and not fail me! Years ago, my H2O pump failed me at Watkins Glen - I was able to drive the 4 hours home without over-heating. (How many Honda's do you know that could accomplish that?!?
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So, is the 944S2 a reliable car? Despite my repair bills from last year (the head alone cost me $5500.00), I would say that the 94S2 is a reliable car.
The car that you're looking at already has much of the maintenance / repair items done. That is a good thing. Given the mileage, I would check the 'rubber' stuff like sway bar bushings, strut tower plates..etc. Also: check the front control arms - the aluminum A-arms are known to develop stress fractures that can lead to catastrophic failure of the suspension. (Don't worry too much about this, unless the car is lowered, or has had a lot of track time.)
The low-end torque of the 944S2 is simply wonderful - it may not have the high-end 'get go' or the tunability of a 944 Turbo, but the 944S2 sure likes to scoot out of those corners!
Hope this helps,
-Z-man.