I would love to do this in my area but the New York City market is really tough with prices that I think are way out of my league.

Also, depending on when you bought them and if they're cash flowing well....might be tough to find deals that are cash flowing as much. 

I am going to learn landlording and I'm going to be as tough as I need to be with tenants. 

All went well until we started to discover things that were not disclosed. to make a long story short, the seller and broker both knew (have proof) of converting a aerobic field into a parking lot and leaving the aerobic system in back of the property which fills up with rain water and leaves my sewage system useless. they literally abandoned the sprinkler and pump system and covered it with material. 

My question is whether anyone with experience of adding similar small interior storage units/lockers can offer any recommendations regarding design/materials/systems?

(e.g. is it taxed the same way, just filed on a different schedule, or are there other material differences?)

I have since found out that they have added additional materials for the flooring job which I wasn't told about and in the contract it says the price is labor and materials included so I don't know how they can add materials and more labor to the job later on.

that's a lot of bread to get the leads.. so you need tons of seed capital.those that have little to no marketing money to start .. pretty tough unless you bag a deal right out of the gate to keep you going.its the same for real estate agents getting started in that industry.. when  I had my shops and interviewed new licensed agents it was something we talked about.. you need ONE years of living expenses in the bank to even think about starting in this business.. same with wholesaling.. to try to do it with no money while it can be done .. flame out is virtually assured because of lack of capital in your small business. 

I have replaced 4 in my time , takes me about 2 hours and $500 in materials .

dirt loans are tough... you can figure on 4 and 12   to 4  and 14 anything less is a good deal.so finished lots are only worth 20 to 25k ???