I'm reaching out to see, if there is any other alternatives we can do that won't damage the walls (painted), or damage the floors downstairs (vinyl flooring). 

I only ask because I managed a profit center for Lumber Liquidators for about 2 years and I was able to get 4 and 5 mm click vinyl for $1.59 before employee discount. 

I've done vinyl plank on stairs and they've come out pretty good.

Based on my own rehabs I would focus on Painting, Drywall hanging and repair, Vanity installation, laminate/luxury vinyl tile installation, ceiling fan and light installation, baseboard and door trim installation, window installation.

In my experience I have seen vinyl plank that is “waterproof” bubble up with not much of a fight.

Laminate, Luxury Vinyl Plank, Engineered Wood, Tile, etc...

If you need it to look nicer, you could grow a vine on it, insert vinyl slats that make it solid-looking, or paint it.

-His units are not renovated:  original kitchens; original bathrooms; one has some pretty messed up carpet and older vinyl flooring; other unit has 4 year old carpet and tile; new furnaces, A/C units and windows in both units 4 years ago.

I would stay away from vinyl due to the extreme uv levels. 

Carpet throughout the main living areas, vinyl flooring in the kitchen and bathrooms, microwave, stove, refrigerator, dishwasher, fenced in patio, and fireplace.