well, if you're dead set on building a turbo 6, which can indeed be fast (turbo anything can be fast).. you might want to go ahead and buy a 250 engine out of a boneyard... the extra 50 cubes will be very helpful. Won't cost a whole lot more in the grand scheme(shelll could probably be had for a couple hundred).
You'd have to set up a pretty much all custom turbo system. The cost could vary quite a bit depending on how much you yourself can do, but it will cost well into the thousands. Remember, you're still going to have to build your transmission , rearend and suspension to handle this power, just like you would have had to with a V8. It might be a bit easier than with an 8 because it is lighter and you probably won't have to deal with the same amount of torque as an 8.
I know theres a guy with a N/A six around here running 14s with all the basic stuff (rear gear upgrade, manual trans, bolt ons, aussie heads, nice tires, etc.), which is pretty good. Might want to go ahead and do that first, see if you're happy... then you can think about a turbo if you're not.