Once again the somewhat elitist view this forum has of the car rears its head ...........
First things first - its a fairly unstressed 3.5 litre N/A Nissan, not a turbocharged Maserati - theres absolutely no reason to be scared of 03/04 imports because they are 18 months older than an 06, likewise mileage over 100K shouldnt really put you off. Just do the usual checks and make sure theres nothing obvious thats wrong.
The clutch is probably more likely to have been changed on a 100K car than a 50K one, if it hasnt the best way to see if its slipping is to put the car in 6th at lowish revs while moving (up a hill is ideal) and then floor it, if the revs rise out of sync with acceleration the clutch is slipping.
An S2K is a better car for 100% maximum attack driving, but in pretty much every other respects its inferior and in some its a totally different prospect, tbh thats pretty rubbish advice IMO, if you want a 350 then buy a 350. Funnily enough a clutch replacement is fairly similar in cost on an S2K too, imagine that
Do you believe buying a car right at the bottom end of the market is a good idea no matter what the car is?
To give this some perspective, Cat C (Still damaged) 350z's sell for around the 4k mark so there has to be something odd going on if a "perfectly working" one sells for near that.
The S2K comment was just my opinion of what i would be buying for that budget. As it would give you enough to be a bit more picky with which one you buy. Yes the S2k is an inferior car but its right in the middle of his budget and the chance of getting a lemon is a lot lower.
Good luck to the OP with what ever he chooses to do