Yes, it can be rebuilt, but unless you are going to leave the truck bone stock im not sure i would bother. Try and find a GM-8 turbo on ebay, or from a builder and use your GM-4 as a core. Its more efficient than what you have now. But, are you sure that the turbo is toast, and it doesnt just have wastegate problems that these motors are notorious for?
For exhaust, SSDiesel Supply, Pinnacle, Jardine, they all sell a pretty cheap 4" exhaust with downpipe. I personally have the SSDS system on my truck, and was happy with the fit and the sound i get out of it.