You will need to pay off the seller's first loan because no conventional lender will agree to be in second position. If you can qualify for a loan of $100K or more, then just have the seller carry the remaining portion of the purchase price (what every you negotiate). You will want to make sure the seller carryback is recorded with the recorders office but if you use a title company or an attorney, then you should be protected.
I can refer an excellent attorney that handles a lot of seller carryback transactions. Just give me a call and I would be more than happy to provide you with his name and contact information. Good luck!
Sean Heideman, Broker / Owner