Seller Carry Back Terms

Hi all. Looking for some advice. Working on a deal to purchase a cash flow property (300 rental storage units), and trying to leverage my $300K to work the deal for the $1.2M purchase price. Anyone experienced in what I can/should reasonably propose for terms? Units currently generate $16K/month in gross cash flow and have a sustained history. I would like the monthly debt service to be 50% or less of GCF. Since I haven’t done a seller carryback before, is it reasonable to propose a 20 to 30 year note with a ballon option at maybe 10 years so I can keep the debt service down? Thanks