I have the same questions. I already have a watercooled turbo I got off a 280ZX, but the H2o lines were not connected. I hear you can get a "retrofit kit" from nissan to put a watercooled turbo onto the V-6, and use parts of that kit for the inline L-28 conversion.
Any turbo rebuild shop can replace your turbo center section with a watercooled housing. Then plumb it into your cooling system and your turbo should last forever. Cheapest place I know of that does excllent work is John Craig's Limit Engineering. A bone stock rebuild of a 280 turbo will cost about $200. His phone number is 1 (252) 232 3194. John does excellent work and can upgrade your unit fairly cheaply. Upgrading to a water cooled housing with the rebuild should only add a few bucks, or maybe none at all!