Good question. I just sold an 1830s mixed use building that sounds very similar to this. It attracted interest from a variety of folks from many walks of life, with wildly differing opinions as to value. This can be disconcerting unless you have confidence in your pricing. There are a couple of things you can do to help yourself arrive at that position and my suggestion is to do both, then see where you are.
First, ask a Commercial Broker for a price opinion using sales data. Since there aren't any recent comps in the immediate vicinity, you cast the net wider and go back further in time, then make adjustments for the variables. The factors you would be looking to match are age of building and any landmark status, size, zoning, and foot traffic, then make adjustments for geographical proximity and recency of sale.
Second, interested investors will want to know exactly how much income the property makes, and may reject the property based purely on the data you provide for them, without even walking through the door. These folks are using the income approach to value, meaning the value of the building to them is determined by what size of bank loan can be secured using the income derived from rents. They may ask for 3 to 5 years of spreadsheets. In addition to the commercial broker, someone who could be very helpful in helping you work this through and produce the financial information would be the commercial lending officer in your local bank. Complete the commercial loan application as if you were buying the building from yourself, then ask the loan officer how much the bank might be willing to lend a buyer assuming the applicant is qualified in all other respects.
As a rough rule of thumb, you could treat the $ value of the potential loan as 75% of the capital value of the building, since banks like see at least 25% equity invested by the buyer in this type of project, and preferably 30%, at least that's the situation around here.
You could also run a second set of numbers using projected B&B income rather than current income to see how much it could increase the capital value. This will also need to factor in the cost of an additional loan to make any structural changes necessary for the conversion. This then helps you to determine the additional value of the building potential. However, if you do decide to price it ambitiously to include the potential aspects, bear in mind that the bank is unlikely to agree anything more that the most conservative numbers.
The difference between the potential bank loan and the asking price represents the amount of cash down the buyer is going to need. Put another way, when you ask someone to pay more because of potential gains, rather than realized gains, they have no leverage with the bank for that part of it. Hence you can only expect to achieve ambitious pricing if a highly qualified buyer with deep pockets can be convinced to buy into a dream, either the one that you are walking away from, or their own version. That's a tough sell.
None of the above is intended to substitute for a proper appraisal by a qualified appraiser., which would be money well spent.