Improving vocals in stereo tracks?

Just got a number of stereo tracks from a friend that I owe favors to. All there is to work with is stereo mixdowns. All are wavs, some recorded digital, some originally cassette (I think). Some of the vocals are irregular, mostly too quiet. A lot already have room or applied reverb.

My intent is to get these tracks to the best possible sound, without much added expense on my part. I don't usually record vocals at all, and typically have multi-track of some sort to work with. So this is not something normally in my wheelhouse.

Technique, applicable software, advice, direction, all appreciated.