It sounds like you have the right engine in the car. All the 7-series turbos other than the very early ones (83-84?) have the water cooled turbo. Up to 1989 it's a Garrett. From 1990 on it's a Mitsubishi. Either works fine. The Mitsu is smaller and responds more quickly, but the overall max power is the same. You *should* have the Turbo Plus kit, which ups your power to 180 or so HP from the normal 160. This is only found, as far as I can tell, in 1989 760T and 780T models.
I don't know what differences are in a K engine other than the Mitsu turbo and it's associated plumbing differences (mainly the extended oil filter and oil return line). All you should need to know regarding parts ordering is that you have a B230FT, and an AW71 sutomatic transmission.
I do know that you should pay very close attention to your oil seals and the cooling system to prevent any possibility of overheating the engine. If you should ever experience high engine temps, STOP and figure out what's wrong. DO NOT drive it to the nearest dealer- even a couple miles with very low coolant can significantly damage any engine.
As for turbo replacemetn, yes, you can do it yourself. A couple of bolts are a real bear to get at with the Garrett but it can be accomplished with normal hand tools and mechanical competence. You will want to soak everything with PB Blaster ahead of time (like for a week) and even then take a chisel to any nuts that give you trouble- SAVE THE EXHAUST STUDS AT ALL COSTS. Breaking a stud means pulling the head, and you don't want ot do that. Replace everything with the nice copper nuts from Volvo and you'll be happy. I expect you can find significant savings by finding a turbo for sale used or rebuilt. Most rebuild shops have a hefty core charge- you may need to return yours to them, in rebuildable condition.
Once again, keep an eye on the condition of those big air hoses because they're so important to the operation of this engine.
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Rob Bareiss, New London, CT 86 244, 87 244, 88 744