In that case, match the freewheel to the hubs on the wheel set you end up with. If you end up with old French threaded hubs, you'll need a French threaded freewheel. Most of the stuff you'll find these days is "English" (USA, Japanese) threaded. The main thing about fitting the freewheel to the hub is the axel and lock nut have to ride out a bit further than the small cog on the freewheel, so that small cog isn't rubbing on the dropout, and there's enough space for the chain to get on and off the small cog.

The dropouts on your steel frame can stretch a bit to fit in a 126 or 130mm hub, but a bike that old is likely currently spaced to 120 mm on the dropouts. A 126mm hub (typical for 6 or 7 speed freewheels) is going to be a tight fit, 130mm even tighter. The rear triangle can be cold set (google it) to fit properly with whatever size hub you end up with.

I've never used a Svelto derailleur, but I know it's pretty old. Anecdotally, every derailleur I've ever tried has been adjustable enough to shift 6 or 7 cogs easily enough, but not sure about that one.

I was recently working on my sister's old 1977 Nomade and its a very low-end, sorta heavy, nothing-special bike. Don't spend too much money on yours, you'll never get it back on resale.