@Champ Leong generally if you are only doing cosmetic things like carpet, paint, and cabinets you don’t need a permit as long as all the plumbing, electrical, and appliances are staying in the same place.

Focus on changing countertops (granite or cheap quartz), painting the kitchen cabinets white or grey, changing the hardware on the kitchen cabinets, and getting some decent used or scratch and dent stainless appliances in the kitchen.

Additionally, there is a basement with a kitchen and cabinets, 1 full bath, washer and dryer and a refrigerator.

That being said, the $15k per unit was more than enough to replace floors, paint, cabinets, and touch up bathrooms. 

I was originally going to paint the walls grey with white cabinets and trim, but after reading this BP article on painting, I decided to go with all flat white:https://www.biggerpockets.com/...Now here is where I (arguably) made another mistake. 

Adding GFIs to come up to code does also....Paint, flooring, cabinets, counters, tile, Etc do not need a permit.I know it's tempting to just skip the permit and try and sneak the work, but I would advise against that.

Some holdovers put holes in every wall, rip off cabinets, steal appliances, etc. 

I would not enclose either window - light and ventilation is desirable in a kitchen, there are PLENTY of cabinets so you don't need the extra wall space and like you mentioned, that would mean patching that opening and matching the exterior siding.

Opened a retails store in the main property where we sell cabinets, flooring, and ACs.

My parents did it that way, then you know  for example, every 15 years plan on new cabinets in the kitchen and that is planned in the CAPEX.