I did just the same thing for a friend. The key is in the freehub body. Here's what I did: I bought an old 8 speed 105 hub and used a allen wrench (14mm, I think) to remove the 8/9 speed freehub body. Slap that on the 7 speed hub and redish the wheel. Wheelset issue is solved. You can get 8 speed STIs on ebay for less than $100. Get a new chain and an inexpensive SRAM cassette and you're good to go.