I wanted to close out this thread with some info on a list that I came up with on how to verify a tank pre-purchase (based on research and y'alls suggestions of course!), that way if it ever comes up in search results there's a conclusion too
so here's the process I came up with to evaluate a tank:
1) seller must have water in the tank (ideally, tank should still be stocked and running, but if not, ask seller to fill with water before you show up)
2) inspect the stand for visible/touchable water, mineral deposits, cracks, rotting, unevenness, evidence of material fatigue. Be methodical (start at one corner and go over every square inch with a flashlight, and most importantly, look INSIDE the stand [I think it makes more sense to start inside first, and then work your way outside, since any obvious issues on the outside would have already been cleaned up or fixed by the seller]). Apply some force on the sides of the stand to determine if it wobbles, etc.
3) Inspect the tank: look at the frame on the bottom, looking for mineral deposits, water, gaps, fatigue (shows up as white on a black or brown plastic surface). Look at the frame from underneath the tank (if the tank is on a stand) to inspect the bottom frame as best you can. Try to locate the manufacturing tag to determine how old the tank is. The top frame must have all center braces present, and they need to be attached to the tank solidly, and they should not have cracks or signs of fatigue
4) Remove the canopy/hood of the tank if possible, and inspect around the frame for chips, cracks, etc
5) inspect all visible seams of tank. Silicone should not be pulled away from the edges, should not have bubbles, and there should not be chips or gaps in the seams. Stick your hand inside the tank and feel the silicone, is it soft or has it turned brittle with time? If you are planning or resealing the tank, then the state of the existing silicone may not be a major concern for you.
6) get a level (bigger tank need longer level) and evaluate the tank for levelness (if this is where it has been housed for the past x years and has been full of water), and bowing. A longer tank will incur bigger problems if it is unlevel than a shorter tank (counterintuitively, one inch of unlevelness over six feet may be worse than a one inch difference in waterline over four feet, since the lateral force from the water will be higher (it accumulates over a larger distance)). Importantly, place the level against the pane of glass and evaluate how FLAT the pane of glass is. If the glass is bowing outwards, or otherwise uneven, it may predict catastrophic failure of the tank.
7) finally, investigate any equipment or extras that you are intending on also purchasing. Ie, if a canister filter worth $300 is being included in the purchase, ask the seller to fill it with water and turn it on to make sure it works, etc.
Thanks again everyone for your help so that I could come up with this list. The tank I went to go see ended up being a no-go (it failed step 2 and 3), but now I know what THAT looks like, so I will be more confident moving forward!
