1. http://www.thompson.com/tpg/ustt.html--The underground storage tank guide. Once it pulls up, you have several choices to play with--if you don't like their stuff, one of the choices is to click on UST Resources on the Internet.
2. http://es.epa.gov/oeca/ag/aglaws/lawspgg.html as a government regulation site, it's pretty dull reading, but should make pretty clear what the requirements are, provided you're willing to spend the time muddling through it.
Just keep in mind that Underground Storage Tanks are a hotbutton enviro law issue these days, and the stuff on the net will cover an awful lot that may sound really scary to you. But you're not talking about buying a place that used to be a gas station or industrial complex--at least I hope not. So in that "one of these things is not like the others" game, you may find that your little tank located on a residential lot for the purpose of providing residential heating requires wading through regulations that have nothing to do with you, in order to find what does. I guess I'm saying be patient while reading . . .
And, last but definitely not least, GET AN INSPECTION and please make sure you have an attorney review your contract to make sure you are protected in the event there turns out to have been a leak.