I'm sure one of the boys will be able to advise you on the likely costs for caliper refurb - on the two cars I've done it on I've done the bulk of the work myself so I can help.
With regard to the 928GTS brakes, two very different but important points:
1. You've got a 250bhp Turbo, therefore your front brakes are the uprated 928GT4 'medium' blacks, with handed larger (than the S2 or 220Turbo) discs. IMHO provided all the relevant working parts from the Master cylinder downstream are refurbed and in good working condition, you have uprated (e.g. AP600) fluid and SS lines & good pad material (e.g. Mintex 1155 or similar), you will NOT run out of brakes even on a trackday. The only caveat I would throw in is, "provided you're sensible and not running slicks...."
2. My understanding is that the Big blacks as run on the 928GTS require an upgrade to 17" wheels and the associated extra costs - you'd be better off putting this money into getting the existing system refurbed and up to scratch !
Just my penies worth but based on experience at numerous tracks (inc Spa & The Ring) in my Silver Rose....