Newbie Questions.....I apoligize in advance!!

1. I do not leave mine in all the time. It really depends on you and what you want to achivie. Some foods I like a char taste. Some I like smoke.

2. I do not use a drip pan but I do sometimes wrap my PS in foil.

3. Thermapen get one.

4. I fill as needed if I can cook the food I have preped with out adding I just light and go.

5. Thermapen must have. If you do a lot of moving your PS when hot good gloves. I have all the AR accessories . I will say it is not cheap but it is great quality and service. Also a digiq or cyberq to help do low and slow unattended. 

1) Generally the lower the temperature, the more likely I'll use my platesetter.  If it's a low and slow, of course I'll use it.  If I'm doing a reverse sear, I'll use it for the first portion of my cook, and then remove it when I open every thing wide open for the nuclear temps.  I normally do pork tenderloins indirect (with the PS), but I've tried it direct, and raised direct.  Direct cooking results in more charring on the outside, and therefore a slightly different flavor.  It also requires a little more attention for turning the meat.  That all being said, there are no hard and fast rules.   Well...except for pizza.  I always use a PS for pizza. 

2) You can always do a clean burn to get rid of charred drippings from earlier cooks, so a dirty platesetter isn't a problem.  If you cook with a dirty platesetter though, it might impart some nasty smoke.  I like to put a little aluminum foil over the PS, since it's cheap and easy.

3) the biggest lesson I learned about temps (aside from calibrating my dome thermometer) was that temps aren't consistent early on in the cook.  Dome will likely read higher than grid for a while, and then as temps stabilize, they get much closer.  At low temps (below 300-ish) any variance of 15 degrees or so really doesn't matter.

4) I only have a large, so I can't vouch for the larger fire pit.  I normally fill the pit, and then pay more attention to how many coals are lit when I fire it up. I've found that it doesn't matter how many coals are in the pit, it's how many are actually hot. I always use an electric starter for 7 minutes, unplug it, and stir the hot coals around evenly throughout the pit.  Once I get the temp where I want it, I shut adjust the draft door and DFMT.  I've left the starter in for 9-10 minutes before, and was left with a raging inferno.  With that many coals lit, it was more difficult to stabilize a lower cooking temp.  Either way, tinker with your lighting methods, and don't expect to nail it the first couple times. Eventually you'll hit your target temp.  It takes a little practice.

5) Welders gloves (to protect your arms/arm hair from hot temps, and to remove a hot PS),  some type of cast iron skillet, maybe a pizza stone for thin crust pies.  High Q grate, instead of the factory installed cast iron grate- it clogs with charcoal too easily, and makes temp control slightly more difficult.

Welcome to the forum!  This place rocks.