Hi Conrad,
I used to own multi-families in Saginaw and have a few thoughts regarding the properties you're interested in buying. Historic buildings cost quite a bit more to insure, if you haven't spoken with an insurance agent as to exact costs you might want to look into that. I've found that Foremost Insurance gave pretty good rates and is easy to deal with. Repairs and maintenance can run from zero a month to about 500.00 on a typical 4-plex... most common issues are plumbing and heating related-expect 150.00 average per service call. Preventative maintenance will save you a lot of money. If you hire a snow removal company, expect to pay a lot. Can't stress this enough.
In a choice between a 2-plex and a 4-plex, the 4 unit is a better pick for this reason: there will be occaisonal vacancies. No getting around it. If you have a vacancy in a 4- unit, the other units will be a buffer against the loss of income. Also, should the long term tenants from the duplex move, it will be harder to find a qualified renter at 800.00 or more per month than it will to find a renter at 500.00 for the 4-plex. If you are considering doing third party payers (MSHDA, etc.) they will generally pay about 575.00 per month for a one bedroom unit so you'd see a nice increase over current rents.
Lastly, are utilities paid for by tenants or owner? If by tenants, you're good to go. By owner, make sure you budget for that. Water will be 100-200 per month, electric pretty low most of the year but expect a jump in summer(air conditioning,fans) and gas is very high in winter.
None of this meant to discourage, rental homes are great investments, but it's important to be prepared. Best of luck!