The taz 5 is a solid machine. Its basically a 6 without auto leveling and with 10mm x rods instead of 12. A taz 5 is essentially a big mini with an LCD and no leveling. Wobble can be a potential issue given the longer spans, but that can be dealt with. Weak spots are the same as a mini, the idler, the small gear on the extruder, pei lifting, the usual. Nothing major. There are a few of the earliest 5s that have the 4 style power supply (rounded black plastic cover) that tend to fail a bit early compared to the others. The small squirrel cage fan on the extruder is also prone to early failure, they use a different style on the 6 now.
Maintenance, basically make sure they didn’t oil or grease the bearing rods, if they did you can work around that.
The openbuilds x and y axis and the anti wobble are popular modifications. The z axis and micrometer barrel retrofits too. Retrofitting to a 6 is another popular path. Itworks3d.com often carries the necessary parts. There are also reinforced carriages, fitting the 6 style extruder to a 5, etc.