avoiding the derail, u should get a tank with a good warranty.a used one could end of costing thousands in damage if it bursts, and it could have all sorts of bad elements.
imo really depends what you have to spend.. and your space needs... we ran into issues w/ out intex pool set-up but we are happy with it now that everything has been addressed. and we are running around 1k gallons for approx. 1$ per gallon. we did lose a few fish during the learning process... so my advise if your going to upgrade and DIY... work slow and methodical and be in no rush ( Its what tripped us up) to be finished.