Sellers Note negotiation

I've scoured this site for the sellers note convos and I got some really good info. I have some terms from a seller that I'd like to negotiate and thought a few of you on here have some really good experience in this. I'm running an SBA 7a. I'm coming in with equity. Enough to cover SBA 10% (probably more like 18%). I also asked for the seller to carry 20% fully differed for 24 months, amortized at 10 years. Payments start year 3 principal plus interest. Interest rate negotiable but at some kind of discount to SBA. I wanted this to give me some breathing room on debt. They came back with 20% amortized over 15, Interest only for two years at 8%, with a ballon after two years. Seller is older and really just wants to get his money and get out. So I totally understand his perspective. It's the balloon of 20% that I'm stuck on. So I'm thinking it would be better to ask for less of a note. 10 or 15 percent with interest only for 24 months, no balloon but I'll amortize that out over 5 or 7 years. Is this reasonable and common?