A key thing you're missing in this particular case is the unrentable unit. You have to use the "gross scheduled rent" when applying the 50% rule. In your case, you said you can get $1300/month for the rentable unit and $950 for the unrentable one. That makes your gross scheduled rent $2250. Then, use half of that, or $1125, as your budget for all expenses, vacancies, and capital expenditures.
I usually use 40%, becuase I manage them myself, and am willing to do it for free. So, that would make your expense budget $900.
You can't just use the rent from the rentable unit because you're still going to pay many of these expenses -- taxes, insurance, maintenance for example -- on the unrentable one, too.
The trouble with trying to identify and budget for each of the separate items is that many of them are very spraodic. You'll only need a new roof every 10-30 years. A new furnace or boiler every 15 maybe. A new sewer line ever 40-50. You may go 10 years with no evictions, or you might do two back to back. All your tenants may move out and leave the property perfect, or they might leave a huge mess. Its very unpredicable. That why its safer to just use some fixed budget.
So, its dangerout (i.e., optimistic) to say "if 50% of your gross rent covers your expenses". Much safer (i.e., more likely to let you survive in this business) to say "I'll budget 50% of the rent for expenses, and accumulate the difference in the months when its less for the months when its more." Then, if you can cover the P&I and your profit with the other half, its a good deal.
This one is a problem for two reasons. You have a lot of work to do, and you may get a lot of pushback from the building department with that work. They're going to be inclined to condem the property unless you can prove you're bringing it up to current codes. At the very least, call the building and discuss the specific property before you buy. Don't beat around the bush. Be very specific about which property it is and what you want to do and ask how you need to do it to get their approval.
The second is that you have tenant who's paying nothing, but incurring all the costs. If her toilet or furnace breaks, you fix it. If she doesn't like your other tenant, she complains to you.