Since it was not a rental in 2020, am I able to list capital expenditures during the year of 2020 on my taxes when I file in 2021? 

It will be beneficial for me to play around with excel and create a mock expenditure.

The S-Corp is not needed in order to be able to deduct utilities, etc.Also - capital expenditures are generally depreciated, not deducted. 

I set about 1% for capital expenditures and 5% for maintenance.

Are you accounting for vacancy, maintenance, capital expenditures, management?

Just depends on your monthly pay vs. expenditures.

Are there any other major capital expenditure items I should worry about for multi families? 

I would put it in a spreadsheet.What is the time to recoupe your different levels of rehab vs what you can expect to net per year based on different levels of rehab and possibly different levels of occupancy.If you can expect to recoup your expenditure in a few years due to much higher pricing and maybe even higher occupancy, then it is worth the investment.

A new AC unit and a new metal roof had me thinking about a reduced capital expenditure expense per month, while the well-maintained detached garage and the large, fenced-in backyard was appealing from a family rental perspective.

Review the lease you are inheriting, put together a budget of what you plan on invest for capital expenditures, study the market for what the home can rent for with the renovations & move accordingly.