Removing the previous tenants that hadn't paid rent or electric in years, new floors, new vanities, new toilets, cleaning the cabinets and shower to make them look new again and repairing all electric, windows, etc.

I have heard people say they placed ads for their potential rental to see how much interest they could generate.

The best idea would be to clean and re-point the bricks (if needed). 

Not sure what you mean here....land is a non-depreciable asset that usually generates a 1231 gain when sold if used in a trade or business. 

It is great to see someone with a high income use that income to invest and build generational wealth. 

What would be the best method, assuming that I am looking at 50,000 properties which could generate 500 gross rent, or 250 after expenses?

Tenant is incentivized to keep the apartment clean, ready to show, grant access, etc.

He wants to buy the property himself but he says needs about 1 year to clean his credit record to get a loan.

In the situation you're describing it take me 4 days for a 2 bedroom 1.5 bathroom unit unless I'm replacing floor covering in which case it's usually a full week and maybe almost 2 weeks.In 4 days, worst case 5 days, I can get a thorough cleaning and painting, inside and out, even removing doors from hinges if needed.In that time window I can also have a plumber replace a faucet or an electrician replace a bathroom vent fan, minor stuff, if needed.

There were a small amount of negotiations around securing tenants in the downstairs apartment at the price they identified and cleaning up some odds and ends (asbestos abatement, fire safety items, etc.)