I own an Early 2008 15" MBP and to be fair i have had no issue with it from new with over 35K hours running. The system pretty much retired for my uses`s now, and my daughter use it for homework and games etc. Only precaution i have made is to raise the fan RPM`s with UltraFan to help keep the GPU a little cooler
Personally although I've had no issue with the Early 2008 MBB, it`s a risk to buy a used one as they do have a known history of premature GPU failure (mine was one of the last few produced) if the GPU fails your looking at a new Logic Board, and they are well out of the extended warrantee period. To me it`s not worth the cost of repair on such an old machine, if my own Early 2008 MBP GPU fails i will just junk it, as the repair cost is almost the value of the machine.
My advice would be to put some more by, and look for a newer version less prone to such issues, if you get a good one like mine it will be down to luck, if you get a bad one it could be dead inside a month...
One last point, i had a corporate Dell from the same period, that had the same Nvidia affliction and IT eventually replaced the machine after four failed Mother Boards, as it was costing more in my downtime than the computer. They told me that the replacement boards were very much more likely to fail as they were just refurbished, not new i cant comment if this applies to Apple, equally it does give an indication on how big an issue and how wide spread. Personally i think Nvidia should have been forced to cover the cost of repair for the full lifecycle of the product, be it ten years or more not just a statutory minimum...