Buying used...

[Nano]

if you can go big, do it the hard part about big moves, is a lot of the water has to come with the tank so you will need a lot of buckets (especially for 90g) also if its set up and you can see that you may not need to worry about leaks too much. when a tank is taken down and purchased I would suggest to leak test it first, but as said, sounds like its still running ATM.
die off is for sure a risk in longer larger moves, in your case I would wait for a few others to pipe in with some tips