Seller Financing Interest Rates

If I seller finance buying a park should I allow about five years for payoff if the purchase amount was $300,000-$500,000 and how much of a percentage rate is everyone using for Interest? Would an interest rate of 4 to 6 percent seem high for any purchase amount? Has anyone ever allowed zero percent for a certain amount of time before interest starts? I guess if I needed to I could always check about getting a bank loan if the park needed refinancing and pay the seller off.