The bathroom have holes, the cabinets have holes, the walls have holes.

If I put $15,000 of rehab into each unit, installing modern rental-grade cabinets, granite countertops, nice laminate flooring, stainless steel appliances, modern light fixtures, new plumbing fixtures, etc., is that worth getting $250 extra rents per unit?

This includes all new kitchens (able to salvage cabinets), bathrooms, appliances, painting, hardwood floors (refinish where possible replace when carpets existed), etc.

Paint is the cheapest and easiest way to make a place look nice.. paint walls, also paint cabinets/vanities and add new knobs will instantly make it better!    

Get out a spreadsheet and itemize everything you want to replace and go to Home Depot and price every single item (count and list every switch, screw, nail, cover plate, bend, elbow, trap, pipe cabinet, faucet, and glue tube).

The custom home has the high quality carpet, soft close Amish made amazing cabinets, 18' ceilings wiht an 18' fire place, etc...the contractor house has contractor grade carpet, the cheapest kind of cabinet you can find with no molding, etc...  

Compare other units condition and consider painting cabinets,  used stainless steel appliances (L&L Appliances on Lawrence), new lav, etc.

It's their job to be functional and form conscious.Make the kitchen nice with fresh countertops and cabinets, have clean bathrooms, paint and re-floor the place if you can build that into your budget. 

My husband is a custom cabinet builder, so there will be a nicer kitchen in there than most other houses around, so we have a feeling we might not be able to ask enough more to get back the expense of appliances.  

The Capex includes everything from HVAC, Roof, Kitchen Cabinets, Appliances, and flooring.