You will find the older homes do require a lot more attention in terms of repairs. As for setting a certain percentage, like Mike said, it depends on a lot of different factors.
For us, in both of our duplexes in St Albans City, our longest vacancy (outside of a planned, one month marathon rehab) has been 4 days since 2013 when we purchased them. There is certainly a demand for apartments that are well managed and in good shape in St Albans City. That said, it is important to make sure the apartment can carry with vacancy built into it. I agree with Andrew, that 15% seems high although if you are taking time to do the work, you might have to plan for these vacancies.
As for PMI, mine drops off at 80%. I haven't used FHA but I have heard depending on the financing, some loans have PMI for the life of the loan. We used New England Federal Credit Unions ARM loan that only required 5% down for our owner occupied duplex.