As broadway said, figure out what your payments would be. there are several sites that run amortization schedules (which is what your payment would be. You put in the loan amount, the interest rate and the term (20 years, 30 years). And you can call your local tax office to find out what tax rates are and what rate they give for homestead exemption. Here they take $25,000 off the home and figure out your taxes on the appraised value of the home minus $25,000 for homestead exemption.
Homeowners ins. is sometimes figured into your montly loan payments too.
Selling mortgages. IT's done all the time, but the buyer HAS to go by the terms of your mortgage. What you sign is a contract and if they violate that, then you can come back on them. Even tho someone else has bought your mortgage, they still have to abide by the conditions layed out in the mortagage papers. But read it carefully because they could put in provisions that say should the loan be assigned to someone else, they have the right to readjust the interest rate.
ASK ASK ASK ASK ASK ASK ASK lots and lots of questions of your lender, your realtor, your title insurance people, etc.
I know that some of what we've told you sounds scary and most people don't have any problems, but it pays to be very informed when buying.