From my experience, a GTR is worth all that money if you can afford it. I personally don't have one, but i've been driving my mates around a fair bit lately and theres just nothing quite like them. You're right to want one over a gtst although i dont have one...well a silvia with an rb20det (which is what i've got) is pretty similar, but is enough power for me. also a lot cheaper to run.
If you drive a lot, you're gonna rack up way too many k's for the poor old thing. Like most people have said, just get a crappy old car (probably a commodore because of their decent size) and have the GTR for when you get home and need to do other non work related stuff. seems to be your best bet.