1) It sounds like you are a bit far out for perfect reception on PPD, maybe you have a few hills in the way. First try one of the GRE or Radio Shack or Antennex 800 mhz ducky antennas, that should give you another bar or maybe even two. If you really want a rooftop antenna AFTER trying those things, I will be happy to point you in the right direction but that is a subject all it's own, so maybe try the duck antennas first?
The radios the units use cost a few thousand dollars and for good reason. They are top notch motorola units and work a bit better than scanners due to a better front end. Also be aware that they have reception issues too sometimes.
2) You should be hearing fire dispatch come through OK sometimes, and really scratchy other times. The transmissions from North Mountain (7th street and thunderbird) should come in pretty well. These will be for north valley calls. The south valley calls come in from South Mountain I believe, and are pretty scratchy for those of us up north. In your location you may not even be hearing those at all. At Deer Valley & 19th Avenue (my work location) the south transmissions are pretty bad. As far as hearing the units it is a matter of line of sight. They don't use repeaters like the Police do. They just transmit from their radios, and the dispatcher can hear it because her tower is a thousand feet or so above the surrounding terrain. There is nothing blocking her signal, but there almost always is something blocking it for you, because your antenna is 5' off the ground as you hold your scanner (unless the fire vehicle is a block away from you). ******Note that Phoenix Fire will be switching to the same system PPD uses soon, they are currently working out a few "details".
For myself, my antenna is on a mast mounted on the peak of my house, and is about 40' above ground level. I can hear the fire units within a 8 mile radius or so, give or take a bit depending on what is between us with their location (trees, buildings, etc.). I still can't hear the ones 20 miles away unless they are up on a hill, or unless a good storm rolls in so the signals can reflect off the clouds.
Outdoor antenna's are beautiful things. No matter how good my handheld scanner works with it's stock antenna or where it is at, it cannot compare to the quality of signal and sound I get from my base scanner hooked up to a rooftop antenna. I have different outdoor antennas including one for 800 mhz aimed especially for Phoenix PD (which also gets the other 800 mhz agencies just fine), and vhf and uhf antennas mounted up high for those frequencies. I can pull in any agency on any band as well as can be expected, most crystal clear with awesome audio. This took hours of work and several hundred dollars of materials, but was worth it to me.
Start simple and get that duck. Then if you want to play further I can tell you just what you need to get the best reception in the valley.