Freddie and Fannie also have loan amount minimums, but even they prefer to stay in the 5, 7, 10, or 20 year hybrid, term with a 30 year amortization. Only HUD offers a fully amortizing 35 year term.
As stated, it's very rare to get a loan that is longer than 15 years, from any lender, and you've done well to get that much.
Generally, the longer the term of the loan the higher the rate since the lender is going to build in a buffer to compensate for market fluctuations. 15 years is a long time in the commercial space.
Grated, a higher rate isn't going to have a dramatic impact on a small loan so the upside to you knowing that you have a long-term fixed rate may be worth it. The second component that affects the payment is the amortization. Are you getting a 25 or 30 year amortization or is this a 15/15?
Lastly, you stated you have 20% to put down. You want to make sure you can actually obtain 80% LTV as that's high with most lenders actually lending in the 70-75% range. An 80% LTV, long-term, loan isn't something most banks would be fine with. Too much risk.