I agreed a price, then a month later it turns out they haven't paid bills that were due, why should i pay them for them.
People owe bills, property doesn't tend to (there may be exceptions, but I can't think of one right now). The previous owners should still be liable for unpaid bills up to the date of completion.
Of course you may need to go to some legwork to convince the collecting company of this, as they don't really care who pays the bill so long as someone does, but legally it's nothing to do with you.