From the photo, it doesn't appear that this condition would be a great candidate for that because of factors like the low height of the window sill, as well as the need to match up with existing sidewalks to be left in place.They make concrete re-surfacers, but your existing slope looks much too great to mess with those.

And I'm wondering what should I estimate as a rehab costs if I needed to do everything - electrical, heating system, drywall, resurfacing floors ... ect.  

I'm in the process of scraping it all off, but it's leaving stains.Ideas on how to resurface on the cheap?

E g rehab on a 1920 built property is likely going to be top tier in expense, but a 1980’s house where you can minimize / eliminate the need to reframe, resurface cabinets instead of replace, etc, etc, this can keep a rehab on < 1500 feet well below $15,000… paint, floors, light fixtures would be typical expense items here, assuming your roof, hvac, hwt etc are all good to go - and they are more likely to be better than the older home.

Had the cast iron tub resurfaced.

A lot of the units need new pool heaters, pumps and AC's and pools need to be resurfaced as they are 15 years old now.

Some of the worst are hardscape resurface, termite treatment (similar for bedbugs), and, slightly less, new roof.  

Each unit has separate utilities and electric.2 units with tenants get all new appliances, new kitchen cabinets/granite counters, new light fixtures and a 'light' bathroom update (resurface tubs, update vanity and light). 2 units that were vacated were completely gutted and redone, including the above mentioned and new shower/tub inserts and plumbing. 

The plan with this property was to make some significant common area improvements (roofs, exterior paint, resurface the pool, add an outdoor kitchen, outdoor TV, redecorate the clubhouse, completely renovate the fitness center, and upgrade the landscaping.

From scope of work perspective I will be doing a heavy up, replacing some plumbing pipes, updating the kitchen and bathrooms, resurfacing and staining the floors which are hardwood, redoing the trim, and painting.