Interior remodel, paint, floor, appliances, fixtures, etc Lessons learned?

Painting and flooring are super easy.

Renovated the basement unit, new roof, new soffit/ facia, new gutters, new exterior paint that was paid for by seller.

We will be installing new cabinets, floring ,paint etc. 

It becomes their home and they can paint the walls any color they want.So, sometimes, it isn't just about the interest rate. 

The tenant fails to pay their last month's rent and leaves the place needing new flooring and paint.

Patch it and paint it.

However I did note that houses are mostly really old, e.g. 100 years or older... any idea if this poses challenges in things like lead paint presence, outdated wiring, rusty plumbing, etc., or have many of the homes already been updated?   

when I first started lending in Detroit in 2002.. the rehabbers there would paint the copper in the basements with grey paint in the low light theives would think its galvanized and not take it.

I will also be doing all the cosmetic things you can think of: paint, flooring, bathroom reno, kitchen reno, so I am open to carpenter or design suggestions.