Fixing up older bikes is very rarely a good financial decision. I do it all the time, but I do it for the enjoyment and satisfaction of "making something mine". You'll very quickly exceed the street value of the bike with a few new parts, especially if you're also paying for the labor. If your question regarding the bikes "not being worth it" is a financial one, you're pretty much there already as soon as you buy the bikes used (unless you get some sort of remarkable deal on them, or they're free).
In terms of components, the Airstream has a number of stated Shimano Altus components, plus Sram Gripshift twist shifters. It doesn't say what the derailleurs are, but they're probably Shimano Altus/Alivio type derailleurs, which is not unlike what you'll find on an entry level hybrid today. '90s components are pretty durable, and this stuff seems to last forever. I have a '95 Motiv mountain bike with Alivio components and it runs great. I have a '97 Trek 750 with STX components and it, too, runs great. Keep things in adjustment, and they'll last a long time. It doesn't say what the Streamline has for components, but it should be similar. It's a GT, so it probably won't have junk on it.
I think you have to plan on a new cassette, a new chain, and new cables from the get-go. At least new inner cables (the cables themselves). You could probably continue to use the outer cables (the housings) if you need to. All that is probably $35-40's worth in parts if you bought on Amazon, and maybe $50 retail at a bike shop. Add an hour labor at a bike shop and I think you're looking at $100, minimum, to get the bike back in order. The bike shop will also recommend a complete tune-up (adjusting brakes, bearings, etc), which will be more money. You might need to replace grips, etc. Doing the work yourself, you could probably do it all for $50, but you'll be at $100-150 at a bike shop. Obviously, you'll need to add a buffer for other unexpected expenses (like if the wheels need to be trued, etc).
Used bikes can be a great value, but you'll be money behind if you have to pay out for a bike that you already know needs work.