RDs as a rule aren't picky, so odds are you won't have to replace it.
There is a 8-spd freewheel, which might be OK if you're not in the habit of bending axles.
7-spd freewheels are still easily available, although brifters are getting sparse.
For 8-spd you're looking at 130 mm o.l.d., or 135 if you're looking at mtb/hybrid standard.