My '75 Sekine has mostly original spokes and I keep breaking them, and it's in the shop again with a broken rear spoke, so I guess it's time to bite the bullet and get the job done. I could use some insight on the most cost effective fix.

I can get the rear wheel relaced for about $60, spokes included, so that would be ballpark $120 for both front and rear.
I'm assuming the freewheel doesn't have much life in it yet, so I suppose I should get that replaced also, so that's what-another $30?

Is there a viable drop-in replacement 700c wheel with the 5-gear replacement (by size) for my freewheel so that I don't have to replace my derailleurs? I think my brakes have enough adjustment to handle the 8 mm difference between the 700c and 27 1/2 in wheels, based on the bead seat diameter.

Here's the components I have now:

-New 27x 1 1/8 tires and spares
-New chain.
-Newer brake pads
-New cables

Original (mostly everything):
Sekine Model SHS 271 21" Royal Blue
Shimano Titlish Front and Rear Derailleur
Chainrings 52/40
Freewheel 14/17/20/24/28
Wheels light allow Araya 27 x 1 1/4 Stainless spokes #15
Hubs probably Shimano Tourney

Tire comparison to newer metric
An objective measure is that of the bead seat diameter.
27" wheels have BSD=630mm (24.8")
700c wheels have BSD=622mm (24.5"), so the former is 8mm larger in diameter.