Need Help with Seller Financing

[Joe Proscia]

I'm currently analyzing a property in Harrisburg, PA. It is a 6 unit house with a parking garage listed at 101k. The seller is open to Seller Financing. I feel that I wouldn't have enough cash to cover the down payment and closing cost on a traditional investment mortgage (25% down + closing), so Seller Financing would be in play. However, I only know the bare minimum of Seller Financing. 

I'm told the Seller only wants to finance 20-30% with a Lender in place to pay the rest. Would that mean I take a mortgage for what I could afford (approx. 70K) and then make an agreement to finance the rest from the Seller? 

When negotiating an agreement for Seller Financing what key terms should I be looking for?

When trying to do some more research on Seller Financing, I thought this would be the best place to get a direct answer and a crash course in Seller Financing.  Thank you for the responses in advance. I'm still getting cold feet jumping into the investment property circle, but feel everyday I come on here an educate myself that I am very close to making the commitment.