Depends on the area, prices can vary. I personally wouldn't pay $500 for that, probably not even $400. Not for used.
A filter for that tank, you can probably spend As little as $25 for a couple sponge filters and airpump or several hundred on a large canister or something in between. I have a 75g tank which is the same size as the 90, just not as tall. I have a sponge filter and an AquaClear 110 HOB filter. It does the job. Two HOBs might be better, might not be, depends on how heavy you stock the tank with fish. Canisters can be better, more options for media, more media, and from what I understand, easier to maintain. They tend to cost more up front though. I just got my first canister but have not hooked it up yet. I will be using it as part of an undergravel setup in a shrimp tank.
FW tanks are generally easy, again, depends on what you do with it. Just fish is easy unless you get harder to keep fish. Plants can add a slight bit more work to it or a lot more work, depends on how into it you get.