Hi Jon, Congrats on your inheritance!
In the past year, 2-br/1-ba homes have sold in northeast Eastmoor (north of Main, east of James) for an average sale price around $50k whereas 3-br/2-ba properties have sold for an average of around $95k. Based on that quick data it does appear that you'll get back your renovation cost in increased property value.
In regards to rent potential, I think it is unlikely that the renovation will increase the rent by $500/mo. A brief look at current single-family rentals in the area show that there are 5 homes right now for rent. Four are 3-br/1-ba and the fifth is a 3-br/2-ba. The 3-br/1-ba's are currently asking between $900-$1100/mo and the 3-br/2-ba is asking $1000/mo.
Have you considered finishing the attic into a bedroom but not installing the 2nd bathroom? That would keep your renovation costs lower but you'd still be able to offer the 3rd bedroom ... My experience is that more renters do want a 3rd bedroom, and of course they'd like a 2nd full bathroom but I'm not sure you'd be able to increase rent enough to cover your construction costs.
My opinion is that it depends on your exit strategy. If you think you might sell in the near future, consider the bath, but if you're planning to hold indefinitely, I'm not sure you'd be able to recoup the cost as quickly as you may like...