No replacement panel will ever make it attractive. 

My smallest project which did NOT need any HVAC, siding, roof, panel upgrade, just small cosmetic job was about $30/SF, with me doing a bunch of the work.

"ok, this is the hot water tank... these are glass block windows... here is the breaker panel... it was crazy. 

@Allison C HendersonWhy not find four 6” circular access panel covers and cover the holes?

That said, there's about $5-6k in roofing, both bathrooms, both floor kitchen appliances that need to be replaced, electric panel needs to be converted from fuse box to circuit breakers and the upstairs furnace seems to be on its last leg.I don't think I have enough to do all the rehab at once, and the roof seems to be holding for now.

He drives by each property on the 1st of the month and the tenant has the check taped in the electric main panel box.

Anyone have experience in installing solar panels at a STR? 

Have you looked into SIP panels or ICF 's ? 

Wood, for 2 reasons1) you can see the obvious wood panels and joints2) It has a raised foundation (not a slab), which they only put under wood frame houses

Here a 6 unit building requires a professional engineer to prepare a drawing with their stamp for the addition of even one 20 amp breaker to the panel.