Its not a big tank so I personally wouldn't put anymore than a couple of goldfish in there.
As it is second-hand, I would suggest you give it a good clean, you can buy disinfectant especially for this purpose in any good aquatic shop.
The air pump sits outside the tank but if you are going to have it below tank height then you will need to put a "Non-return valve" in the airline. This will ensure that if you have a powercut or the pump stops working, then no water will return back to the air pump and cause it to stop working.
And if you are having just goldfish, then you won't need the heater. As I said before though, please do read up about it, you really need a filter too, a visit to your local aquatic shop will probably be the best thing to do. Tell them what you have and they will advise.