You have an issue that needs addressed FIRST. In the early 90's the mfg's were changing the spacing so you need to know what your rear spacing is BEFORE you go buy a new wheelset. The rear spacing between the dropouts can be either:

126.5mm (commonly called 126 mm)
130 mm
or
135mm


If you have the early 126.5 spacing - good luck finding decent hubs. Everything I've seen is JUNK.

If you have 130mm spacing, use an aftermarket road bike rear hub (like the Hope Pro 3) and have wheels built. If you go this route, use Mavic 517 rims as they are ultra durable and these rims can be used with rim brakes. DT Swiss spokes of course.

You will be upgrading to a Hyperglide cassette and 7 speed cassettes are still available in most tooth configurations. You may have some issues with shifting in the middle ranges of the cassette as your freehub is UniGlide and the cassette will be Hyperglide and there are some miniscule spacing differences.

As was said, you will need the 4.5 mm spacer behind the cassette with the 7 speed. You can get one at your bike shop and they will know what you need and can tell you where to stick it. ( )

If you have 135 mm spacing, you can use any new wheelset on the market with a rim which can be used for rim brakes. Google to find the mfg's websites and learn which rims are OK for both disc and rim brakes. Ceramic rims need special ceramic pads (which are more expensive).

It is my belief that, since you have a freehub setup, that your spacing is 126.5 mm. You would be better off replacing the rim/spokes only as this would be the most cost effective way to go. Provided of course, that your hubs/bearings/cones are still in good shape. if you need hubs you're better off replacing the entire bike.