Doing a live in BRRR of a two unit and ripping up the old green shag carpet . 

-does the interior need cosmetic updates - paint, carpet, lighting, or even heavier cosmetics - appliances, kitchens, bathrooms?

Is there carpet ? 

He will install new luan on top of the plywood that currently has carpets in order to make the floor smooth. 

I took extensive video and pictures when i moved as the house was not, to most people in move in condition, obviously cleaned but very cheap and very old carpet, large areas of rooms that weren't entirely painted weird hardware sticking out of walls, screw holes or remaining screws and nails, several patched but not painted areas, tons of areas with drawing, water, oil marks on the walls (from a centimeter to 1 foot) etc etc! 

I was thinking I'd do carpet upstairs for the tenants comfort.

For example, you find a space with good floors (LVP, tile, pretty good carpet - can be cleaned), but needs a paint job and some granite/new counters.

@John DeanJohn, I’ve used both Kilz original oil based (on any surfaces that are real bad/ deteriorating) and then their newer product that is water based called Restorer... most recently (April) I bought a place that had two beautiful dogs that literally never saw the outdoors and after stripping the carpet rolled Restorer on the entire subfloor before throwing down the usual LVP.

It replaced tile and carpet. 

We'd just installed new carpet and found out a week or two later the tenant had spilled fingernail polish on it and left it to harden.