The tax liens will not affect your transaction. The current owners or the bank are responsible for the taxes on the property. When the transaction closes, they will just pay the amounts due on the tax lien. I have dealt with quite a few foreclosures and short sales that had tax liens on the property. They will all get paid at closing. Hope this helps!
Taxes are always paid off first. The total amount of the lien(s) should be noted on the HUD1 as a seller expense. It shouldn't make much difference to you as the buyer. The only change will be who gets the money designated for the lien.