This discussion has come up over at NECO Forums and here are my thoughts. V6 vs. I4 turbo buildup:
V6 Pros: potential for more power and more intial power/torque
V6 Cons: 1 more head 2 more cams 8 more of every valve component 2 more pistons 2 more rods 2 more sets of rings 2 more sets of bearings 1 more head gasket
All the components listed above are those that would most likely need to be replaced in preperation for a turbo application.
V6 summary: More parts to replace with aftermarket parts. Cost of doing so would be enough for an I4 buildup + turbo kit. Anyway you look at it, the I4 is much easier to work on and less expensive as well!