There is no way for a technology to accurately comp a vintage 800 sq foot 2 bedroom  rental vs a 1500 sq foot modern 2 bedroom rental side by side. 

At first brush, take a look at the dollars per unit for multifamily sales for that vintage of property, and compare that to condo sales of similar vintage and see if there's a delta. 

Real estate investing is hard work beginning with the search for the hiddden gems.  

That could definitely work for him too.Although, because there's no HOA, just make sure the other half doesn't decide to paint their property a different color.Came across this "gem" of a Gemini once on my travels:

If you have any quick tips, gems of knowledge, or something like that, then I will appreciate that and I hope others will too. 

This will highlight the true real gems and expose the fakes.

After we demo-ed the 1920s-vintage 3/4 in. tongue-and groove oak and got down to the planks, we screwed 19/32 in. ply into the joists with 1/4 in gaps, filled the gaps with elastometric floor patch, laid down 1/4 in.

I would have only sold those gems if they were on fire.

We have grown to love this house, and after 3 years of total renovation, we have turned an original 1920's 2-story home into a 'one-of-kind' gem.

Although I personally think you can find gems in places like North Redondo Beach and certain parts of Torrance.