Hi there and welcome to AA!
As for buying used items, it's definitely OK. Make sure you inspect the tank and stand before you purchase your items. For tanks specifically, inspect the braces as they are structural and not just for looks in larger tanks. Be wary of folks who use Vaseline to make scratches disappear in the tanks they're trying to sell off.
If you're going for a reef tank with corals, you really should use Live Rock. LR
+ circulation is the best filtration for reef tanks.
Check out the articles on AA in regards to starting off your tank: