We have an UGF on our 20 gal high. Only reason we have it is cause we bought a used, established tank and it came that way. I have done a fair amount of research on the UGF filters. They do work fairly well, but require regular, consistent maintenance. And if you have any bottom dwellers, you almost HAVE to over feed a bit so that they have a chance to eat. When I drop in sinking pellets for my black kuhli loaches, you can watch the filter slowly suck the food into the gravel as the water breaks the pellets down. They have about 10 - 15 minutes before most of the food is too far into the gravel for them to get at.
I can also tell what areas of my tank are the places I feed the most often. Every week when I do my PWC and vacuum my gravel, there are places where a lot of junk comes out of the gravel. Occasionally I run the tubing for my gavel vacuum down the UGF uptake tube and under the plates to get any junk down there. Its kind of gross what comes out sometimes.
We do run a HOB filter in our tank as well. I would never use just an UGF. If you are willing to maintain it, they can work, just be ready to clean your gravel frequently.
Here are a couple links to articles/discussions on them.
Using Undergravel Filters to Their Full Potential | That Fish Blog
Undergravel Aquarium Filters
Freshwater Undergravel Filtration, Tried and True
There is another really good one that I am having a hard time finding.
As for adding one to your existing tank. We changed our gravel substrate at one point. We were already running an HOB, so I wasn't to worried about losing the bacteria from the old gravel.
We put all the fish and about 70% of the water and all the decorations into a large plastic tub. I then scooped out the old gravel into a bucket, took the tank and rinsed it out, put the UGF back in, put the new gravel in, then replaced the water, fish and decorations. Our fish were stressed for a couple hours with the moving around, but we didn't lose any of them. It can be done.