If the property were placed into an Illinois land trust, with a neutral party as the trustee, you could set up an occupancy agreement where
#1The existing owner keeps his financing in place.
#2 you have almost all of the rights as a homeowner (tax deductions, rights to dispose of property etc.)
#3 the only way you can lose the property is if you do not make payments (at which point you either pay off the property or it goes back to the original owner.)
#4 properties in a title holding land trust are not subject to the Due on Sale clause found in all mortgages
To help the seller have more confidence in you, you can place say 3 months payments with the attorney/company that is the trustee. Then if you get behind, the seller is given notice, and he doesn't have to scramble for money to keep his credit from getting tagged. If you do not pay, the process to remove you from the property is not foreclosure. It is eviction.
There are quite a few companies around tha to do this and will manage the trust for a small fee each month, but a good real estate attorney can do it just as well.
(Trusts are how the investor group that I work for structures all of its private mortgage financing)