give us some details - what is in the realm of possibilities on the deal?
Variables include down payment amount, interest rate, simple or compound interest, term, does it amortize or is it a balloon payment, your credit, your balance sheet, the seller's motivation as to why finance it versus insist on a cash sale.
Also, what is your motivation? Buy and flip, hold long term, etc.
How easily can you refinance this in the future? Prepayment penalty concerns too?
You can call some hard money lenders, and also some more conventional lenders and your bank to get a range of terms for investment property purchase.
I guess it gets back to why the seller wants to or is willing to carry paper, and why this is something you'd want to do as well.