They ONLY thing you'd be paying off there is the back taxes - county, school and local municipality. The don't check the liens for you. You would have to pay off any and all mortgages and all other liens. There could be liens for water, sewer, garbage, gas, electric. Even the IRS could have a lien on it for back income taxes.
PA is a tax deed state, so you could get the property by paying off the taxes and owning it free and clear. However, no major lien holder is going to lose their inetrest in a property because of a tax lien, so they usually pay it off so they can maintain their priority lien holder position.
Additionally, there aren't many owners who are going to lose their property over a 20k tax lien if their home is free and clear. Usually, they'll find a way through refinance or friends and family to pay ity off. That doesn;t mean you can't get a great deal, they are just not a dime a dozen.
Also, keep in mind there are investors who make a living going to those sales, so you will have competition. I recommend doing some homework and going to a few auctions. It will ultimately pay off for you. The site below just gives some general info on tax sales in PA. Hope that helps.