If you are valuating SFR's focus on  other long-term plays like debt pay-down, equity, and tax benefits...Not discouraging your valuation..practice helps...but your vacancy loss is pre-determined as a component of GOI...it's not an operating expense...and capital expenditures are what they are...again, not an operating expense (and 15% may be excessive...).PM me or feed me a couple of your property addresses and financials and I'd be glad to help you sort these out. 

Also, the expectation in these units is very basic appliances and features, so the overall costs of capital expenditures is very much on par with the rest of the market.  

When doing the numbers for calculating cash flow, remember to account for vacancies, capital expenditures, and maintenance. 

Also, in general, is there a specific rule for what constitutes a "repair" and what is a "capital expenditure"?

This heavily depends on what type of property it is, but I have a client group of hotels where whenever they place a hotel in service, cost seg lets them pull several million into de minimis safe harbor expenditures.

Then, how much of the rent will disappear on the mortgage AND holding expenses (property management, vacancy, maintenance, capital expenditure, insurance, taxes...)?

Appears you are planning to rent it.If so, then delay any major capital expenditures that won't have an impact on rental income.Best to do major capital expenditures, if possible, right before you sell.

If you have a lot of repairs and capital expenditures it could push you into the red.

Those are attractive numbers, and I know a good property manager who could handle things on the tenant side, but I am concerned about getting hit with big Capex expenditures on a property that's not worth it.

In all of the projects I look at, virtually no one sets aside sufficient funds for capital expenditures.