I would just wake up early, have an extra cup of coffee, and redo the tank entirely in one day. I would keep the plecos in the tank for now - until you can find homes for them. Check to make sure they are not emaciated, they are often so with people who buy them for "cleaning" purposes.
However, I would not consider doing live plants or sand, unless you are planning on doing monthly maintenance on the tank. If so, then ignore my suggestions...but if you're redoing it for them, and then having them go back to caring for it on their own after that, this is what I recommend.
Personally, I would say to keep the UGF. They are probably not going to be able to maintenance a filter, realistically. Unless you plan on doing it for them, consider that a UGF doesn't need any media replaced...and it can go a lot longer without being serviced without disastrous consequences. Sure, a dirty gravel bed's no good, but it's a lot better than a filter that has stopped pumping and leaves the fish without any oxygenation or circulation. I would recommend you switch to powerheads (if airstones are in place, currently) and use plain old pea gravel from the garden center, rinsed several times.
I would keep 50% of the old water, no matter how yellowed it may be, due to the TDS issues above. Running some fresh carbon would be an excellent idea.
Those rotting plants aren't doing anything to help with the water chemistry, and the low light plants that will grow under two weak fluorescent bulbs won't do much to help it either. Stick with Java moss, only, along with maybe Anubias perhaps (although I imagine even those will have trouble) and use artificial plants, they make some plastic ones that are not bad to look at. Keep their shell decor, because they probably like it, and maybe try and get other things alone those lines. Driftwood would not be my first choice unless it is not going to leach tannins, and it almost always does. This tank is already acidic, and will only get more so with the neglect, so anything that acidifies the water would not be my first choice.
I would recommend maybe even adding some crushed coral...but you may see a diatom bloom as a result, unfortunately.
Go with resin ornaments, artificial driftwood, etc. I would say. Stick with bulletproof tetras like black skirts, hardy livebearers, maybe some barbs (though not cherries) and zebra danios. They want a community tank, I'm sure, so just think of the hardiest fish you have available and stock it with those.
This tank will never be perfect, or anywhere near. What you can only hope to do is damage control! Don't expect them to change, they are much too old for that.
