Converting a cellar to a basement could be costly with little return, but I can understand your concern. One of the things I tell my sellers is you can't worry about something until it happens. Because of prices in Sellwood and the rental market, you may attract a cash buyer, and conventional financing guidelines may be flexible enough. Buyers also have the options of renovation loans.
The benefit of working with an agent in a situation such as yours is mitigating any inconvenience to your current tenants (happy tenants stay which is more valuable to an investor buyer), and we incur the expense of marketing, etc. It sounds like your current primary concern is you decide to sell, get into the inspections process and the buyer says they will walk if they don't get prohibitively expensive work done. You can decline their request at that time, and they can take it as-is or walk. If they walk, you move on to a (hopefully already placed) backup position, or don't sell and you are back to the status quo.
All concerns are justifiable, but luckily most are also fixable. I'd be interested in taking a look at the duplex and getting an idea of what your goals are to see if there's any way I can help...
Licensed Oregon Broker