Make sure to attend the local REIA group if you want to get knowledge and access to some off-market deals.

Now, she can't stop smiling and asking me "how many houses do you own?" 

Besides the obvious that your place could get gutted and cleaned out of anything valuable, there are a number of scams where your listing gets posted to Craigslist, someone else gets access (IE via your lockbox) and "rents" your property to unsuspecting victims.If you do collect fees electronically, don't use Paypal.

I don’t have any other hesitation other than the total income they receive from their jobs not being sufficient (at least 2.5x the monthly rent), but should I consider the six months advance rental payments knowing they will have access to additional income made from the sell of their home?

They can allow you to access markets outside of your own, which can be invaluable.

There's no way I'd own more than a couple of houses without it, as the ability to leverage would be so limited that only the cream of the cream would have access to those funds.

The purchase price is 399k.I am working with a partner and we have access to 150k, but not 400k.

You can look for someone who specializes in selling that product but they will most likely be a Farmers or Nationwide agent that has access to quote with those carriers. 

Can anyone recommend a low cost brokerage in Massachusetts where he can renew his license and maintain MLS access?

Also, most window companies change their designs and stop making replacement parts on a regular basis.