Does the letter specifically have to be about guns? Your family doc won't write you a letter stating you are of sound mind and body and not currently on psychotropic meds?
If you just walk in off the street and meet a doc, they generally don't want to write something like this because they don't know you from Adam. You need someone that had a relationship with you (quick and easy) or you need to go through the expensive evaluation process.
No down side to LP/NG imo. First time I didn't have to go outside in the pouring rain to add gas to the gen. was all worth it to me. LP of course is a bit different but the tank swap is pretty quick compared to opening up the generators tank and trying to add gas in the dead of night. Note here...Top off your tank before the sun goes down. Even if it means getting soaked. Most people (at least us) did turn off the gen before we went to bed. You can top it off in the morning. Also don't ever let it run out of fuel. The power surges will reek havoc on both your genset and everything it's trying to keep running.
Speaking of rain...I bought one of these to keep mine relatively dry in the rain. Pretty nice cover if you don't have someplace to put your genset while it's raining. If anyone decides to get one hit me up for some assembly tips.