In terms of paying off the mortgage, you’d have to work directly with the lender to make sure they get paid and that they discharge their mortgage. If you’re unsure about how the process works, then honestly – it probably is best to just work with a title company. It may cost a few hundred dollars more – but for a property like what you’re describing, that’s what I would be doing anyway (I use title companies for anything in excess of $10K). They will take all the guesswork out of this process and make it WAY easier to get the job done.
You should work out a budget for advertising, because it can get pretty pricey. Although it’s not mandatory, it’s a good idea to get a selection of photos of your house showing it at its best, and using these in your advertising. Make sure any advertisements include accurate details about how and when you can be contacted. If you don't want calls from agents offering their services you can include the words “no agents please” in your ads.
You can also look to certain companies and organizations that have a concentration of people that may fall in the above mentioned roles such as investment companies (i.e. Charles Schwab, Fidelity), financial advising companies (i.e. Morgan Stanley, Merrill Lynch). There may be a higher concentration of people who can both afford and want to build their own custom homes or to build income-producing properties. Try linking yourself to financial advisors who may already know of people who have these types of dreams. This isn't a comprehensive answer of all the possibilities but just some thoughts. Good luck!