I think you have a good idea. The details are a little tricky. The tenders came in different sizes and were coal or oil fuel. There is a lot of unmet interest in oil tenders, I would like to convert three of my engines to oil burners and think Vandy tenders would be nice.
If you sell the whole tender and not just the shell one common size with two four wheel trucks could work, you might also consider a "long range" tender with two six wheel trucks.
If you are just going to manufacture a shell build it to fit the most common sizes of tenders.
Design your product to easily accommodate existing electrical control systems. (DCS Legacy)
Figure out someway to make a lot of different road names (maybe BTO) and have an undecorated version with just the safety and volumes lettering.
Price range is tricky, for a cast metal scale detailed tender $150.00, a plastic scale detailed tender $80.0-$100.00. A well made shell $80.00.
Douglas