Replace the roof and get decent coverage assuming other area have been updated (electric, plumbing etc.)

Roofs, Plumbing, Electrical,  HVAC/Furnace. 

I can't speak for contractors, but as a DIYer, I know even the most basic kitchen renovation may end up costing me 10k if I were to do it myself. 2-3k for cabinets. 1k for plumbing and fixtures1k for electrical and fixtures2-3k for granite countertops1k for tiling/flooring1k for miscellaneous like painting, etc. 

Plumbing Inspector came out and failed the plumbers for some work they did and also dinged me on simple repairs/updates I was doing in the house.

New sewer, electrical, electrical panel, mini split, tankless water heater, plumbing, flooring, kitchen, bathrooms, turf, driveway, hardscape, etc.

Is this just a cosmetic rehab or are are there problems you have to fix in electrical, plumbing, utilities, etc?

Since it is going to be a rental, first concentrate on any items that need maintenance immediately that prevent it from being rented now (creating cash flow)(including all parts to be working, such as HVAC, plumbing, electrical, etc.). 

The age, type, and condition of the plumbing is a significant factor in whether I'll buy a property, and how much I'm willing to pay.Electrical.

Hi Michael,It is great that you are doing this together, I read some great tips on here, something I wanted to add:I have been checking a lot of 2-4 units, in the past it was not easy to find them in good areas.During COVID, a lot of investors decided to build these types of units which is great, but not all builders are great, I had a client, he purchased 2 duplexes in the same area, we had some pricy maintenance issues, one was with the plumbing.

Water heaters, furnace, plumbing leaks, replacing a roof, replacing the main water line, tree trimming, microwave replacement, etc. $20-25K would be awesome but you can likely get by depending on condition for $10-15K.