SunTour was great stuff, although very costly to upgrade because you basically need to replace a lot... I've done tons of these bikes. If the front shifter is working, you might be able to leave that for a bit, but reality says that SunTour is not compatible with anything outthere. So it means that if the drivetrain is worn out now or if you wear it out later, you'll be looking to replace shifters, derailleurs,chain, cassette, rear wheel as the cassette is made for SunTour cassettes and does NOT fit anything else. On the other hand, maybe you'll be lucky that someone didn't ride it all that much and you won't need to do anything for a bit. The Giant Iguana frame was pretty decent quality, so if that's in good shape you might be lucky there... but in the long run this will be a very expensive bike to keep. Another thing to keep in mind is that it is a 90's frame, so that the top tube will be fairly long and may not fit you very well... and you may need to spend money there in order to try and shorten the reach a bit.In the 90's many bikes were freewheel rear wheels, which often times would mean that the freewheel would simply break off the hub.... so something else to think about as well.It sounds as though you work at a shop, so perhaps you'll save some money there.
In the long run, I wouldn't recommend you buying a bike that far back simply because at some point it's going to cost you quite a bit... if you're working in a shop get something that is more up to date whether new or used and you'll be able to maintain over the next few years much easier.... and you will have many more options for fit.