It has little to do with the Suntour rear derailleur and a lot to do with the amount of cable pull required to move from one cog to the next on your 7-speed setup.

There are no 7-speed STI shifters, so the pull on each index will not move the old Suntour derailleur enough (likely) to shift cleanly from one index to the next. It would be fun to try to get working properly, but I imagine you're going to run into lots of problems. The first STI shifters were Dura-Ace 8-speed, and those have/had a different pull than all other Shimano STI shifters. One might work, but expect you'll either need to model it thoroughly or spend a lot of time buying parts on eBay and then selling them when they don't work.

If you're going to go to a modern drive train, I'd recommend going all the way, which would include a hyperglide cassette and freehub assembly on a 130mm wheel.