I think it depends on what you define as acceptable as far as your investment criteria for returns are. 

You have to pay the interest monthly but if you do the project quick enough then hopefully it's only like $2500 a month depending on your interest rate.

This is not some mysterious questions with it depends answers.It's just math.If you pay of your house you avoid that 4% interest over 30 years. 

@Ricky Jefferson It depends on the level of complexity and what you are really looking for in a bookkeeper/accountant.

In terms of what you can build-that depends on what zoning allows.

Depending on which, the treatment is entirely different. 

I guess it would depend on your stock market skills, but assuming you know how to invest properly...

@Mark ZarragoitiaThe relationship and fees will depend on the type of advisory company, but having an expert on your side is critical when investing in non traditional assets. 

I find that this can be a little subjective depending on what criteria you follow such as the 2% rule for example (so far, no multi family properties pass that rule but then again, markets are high right now).

Also, you may want to seek our private lenders and/or hard money lenders depending on your end goal and needs.