1-swap freewheel to 7-sp. No change in rear width, pick up 1 speed and STI compatibility. Get an indexing freewheel.
2-7sp STI's or the cheaper and generally more available 8sp STI's.
3-DT cable stops and some STI housing would be needed.
4-Generally the FD will rub a bit due to differences in cable pull, but you can "move" the rub to where it's least used.
5-If you're staying with 7-sp, you may be able to use the same RD, and I'd certainly try it.
Note: if you go with 8sp STI shifters on the 7sp setup, the 8th speed will just be along for the ride and do nothing.
Like the US, in some ways....