The 951 can be a reliable car as long as you find one that has been taken care of. The key is to keep up on regular maintenance (plan on about $2000/yr for one that is in good condition). Same goes for the 968.
To get more power out of the three cars you listed, the 951 will have the biggest payoff. With $1500 in modifications you can have 260-300RWHP and 300-330RWTQ... dyno proven! The 968 won't get much above 220RWHP no matter what you do to it short of forced induction, which will cost upwards of $4500 to do it correctly. The 350Z is in the same boat as the 968. You won't see more than 240-245RWHP. I know this because I have seen several 350Z's on the dyno with all sorts of aftermarket intakes/exhausts and none of them make more than a few HP more than stock. Some actually lost power.
As far as I know, nismo does not make any products for the two porsches you listed. Firdenza makes a lightweight flywheel for the 951. For exhaust, check out
www.speedforceracing.com - One of the most respected 951 tuners around.
Hope this helps.