If the rest of the group matches the crankset (Ultegra) pictured then you should need a Shimano 9 speed cassette.
Max low tooth for that year Ultegra is 27.
11-27 would be the cassette ratio to shop for but you don’t have to get Ultegra.
Base level Shimano HG50 in a 12-25 range is relatively cheap and will work just fine.
Or if you can track down old stock Shimano 105 in 9 speed to get the 11-27.
Take a picture of the right shifter and I can confirm for you.
Does it actually shift?
Problem with shifters that old will be that the grease for the return spring hardens up and will need to be broken down with some T9 or clean streak before it will grab shift points again.
Wheel size is 700c.
Most wheels you get these days come with spacers to reduce down to 9.
New entry level pairs can be had for $75-$125.
Buying used is all over the map.
Just make sure they’re true and the hubs still feel smooth when you grab the axle.
Still, spending $5 for that specific bike is pretty good.
If you don’t want to put $200 in parts and a good tune up to get it rolling like it did in the late 90’s then, find a guy that’s 6’3”-6’5” and sell it to him for $20. lol