Last year on one cabin I had to replace the roof, hvac, & tile shower which came out to $22k.  

I would focus on getting that househack first.You can take advantage of a lower downpayment, get a roof over your head, and an investment property all in one.

I sold a beachfront home last year that was ICF constructed in 2000 with a metal roof on concrete pilings and the insurance went up triple what my larger 2019 stick built with shingle roof 6 lots down is. 

With your estimate, you might earn $800/month for 2 years and then incur a vacancy at the same time as you need a roof replacement and for that tax year your cash-flow might be -$1,200/month.

If you are buying a house of any kind with mechanicals and roofs, you need reserves.

There's no roof or sewer for you to manage.

Is this a decision about one or the other - either write off the old roof OR depreciate the new roof? 

I just closed last week on a duplex, new roof, windows, siding, furnaces, HW tanks, all cosmetic for ready,.....70k, Rented for 1800.

Has a unique garage that is open air, but  roofed and on the same slab as the  rest of the house. 

However, the homeowner insurance coverage can bit steep at times due to the natural disasters down in FL.