Even with "good" paint, I typically cannot wash away all of the grime once a renter has left and at least end up repainting common areas.

If you're going to repaint regularly I'd prob recommend going with flat over Eggshell as it hides any imperfections better allowing more forgiving drywall work/repairs.

We're going to repaint and rearrange some of the bedrooms to accommodate more to families but don't want to overdo it either and take away some of the older crowd that rents out for their kids/grandkids.

From my experience, I have learned repainting a building can immediately upgrade the exterior of a building.

Flood insurance is a killer, the property's living room furniture will convey as well as an agreed upgrade in kitchen appliances(fridge, stove, dishwasher, oven), and repaint the entire interior in agreeable gray .

Some people avoid colors that show marks and dirt easily, but regardless of the color you'll most likely have to repaint when the tenant moves out anyways.

I repainted the inside owl gray, and the exterior white with black highlight.

However it does have a leak and I assume some mold and the previous owner cut out the leak spot, repainted and sold.

It's also bad for your reputation if you're running around town making $200,000 offers on homes and then after inspections trying to renegotiate to $170,000 because you didn't understand the cost of repainting the interior of the home or installing granite countertops before you made your offer. 

You can also take a bit more time and actually find out how much it would be to replace the floors, repaint, put in a new roof or whatever else you think the property needs.