I don't know how long it has been on the ceiling (the bathroom has a shower, operable window but no vent).

The main reason for wanting to make it a 1 bed is because there is almost no natural light in the living space at the minute.The windows on the non-bedroom walls are so close to the neighbours property that they hardly let in any light at all.

I buy in good neighborhoods (Two tests: There are no bars on the windows and I don't feel the need to carry a gun.); in Houston area the houses are $135k to $175k ARV; the monthly rent is about $200 more than 1% of the TOTAL COST. 

Also, for long term buy and holds, you will likely move some of the bigger repairs/renovations, (windows, roofs etc) to the basis as opposed to taking the full expense in the year...

Tenants often expect to be compensated as most landlords now provide a gardener to make it not look like a rental.The basement window needs to have safety latches to secure.

Former tenants were using A/C window units.I'll be seeking some estimates/bids in from contractors at the beginning of the week, but wanted an idea of what to expect cost wise as well as the see if there is a good rule of thumb to keep in mind when looking at future investment opportunities. 

On the exterior it's mostly concentrated to window/door trim and on the inside there are two hallways with hairline cracks in plaster which I was informed by one of the contractors would only be filled with Ready Patch.

It can easily be converted to a 4/1 because the rooms are already framed out, just need to change out the windows in each room and make them egressed per code.

They sound like older homes, so how are the windows? 

One lease, one rent payment, one security deposit.If you deal with them as if they have different leases, then if there is a broken window, you have to figure out who to charge for damages?