The buyer is responsible for all taxes, maintenance, and upkeep, so it is mostly on the buyer.

Is it better to use fiberglass or rockwool insulation to insulate weight cavities before putting in replacement windows?

The only reason I might consider doing it the other way is if the tenant has been abusing the appliances or they have a lot of problems with appliances and you cannot afford to continue the upkeep of them.

A good property manager is worth their weight in gold.  

We are now coming to the end of the lease and we want to increase the rent to the original amount we had determined to be FMV as well as what we personally needed to offset the mortgage/upkeep costs. 

My goal is to put together a portfolio of 100 single family homes by the time I retire, and pass them down to my kids... so I'm looking for properties that will make the most sense LONG TERM in regards to minimizing upkeep, capital expenses, insurance costs, etc...

Just remember, the credit agencies keep all this stuff hush hush, and everyone is weighted differently, so no one here can really tell you want YOUR credit will do, under the circumstances.You can use the heck out of your credit cards, IF you pay them off within 30 days of the charge, and "technically" shouldn't get hit with utilization problems.  

I have not researched it, but I would question why it won't hold what can't be more than a few lbs of weight.

Some receive a government check and can't afford and apartment, some have temporary jobs with unstable income, some can't get utilities due to past mistakes or can't pay the deposit, some can't afford to furnish apartments, and some don't want the cost of monthly upkeep of a house ie lawn care.

That would have added benefits such as depreciation and upkeep deductions.