Congrats on the acquisition of your Smoke Vault, and welcome to SMF!!!
If the 18 performs like the 24, there isn't a whole lot that needs to be done with it. You can grab a few accessories as you mentioned, based on your comfort level and experience. I actually have used up to 3 meat probes plus a pit probe (grate temp) in my 24, depending on what I have soaking in the smoke.
The only real mod I did to my 24 was to bend the tabs on the lower side intake draft controls so I can close them all the way, or anywhere between closed to full open...this improved my grate temp variations, along with sitting a 24" vent stack on top to improve the draft through the burner housing. The intention of this whole set-up was to not allow cold air into the smoke chamber...make it all get heated before entering the smoker chamber...overall, it seemed to help quite a bit. The 18, being a bit more narrow cabinet, may not suffer much from grate temp variations, and may perform similarly to the 3405-GW GOSM I started smoking with 12 or so years ago.
I haven't looked at an 18 for so long (maybe 5yrs) I don't even remember if they have side intake draft controls...
Oh, you can calibrate the door therm with a 7/16" end wrench (if I recall correctly) on the hex portion of the thermometer stem...just turn it the direction which makes the dial read more correctly. Use a probe or oven thermometer of known accuracy placed close to the position of the door thermometer stem while in operation...best to calibrate at close to the chamber temp you want to run it at. Also, verify grate temps in comparison to the door thermometer while loaded and smoking, as they will be a bit different. You can then refer to the door thermometer for approximate grate temps.
For thermometer calibration, elevation effects the temp of boiling water...here's a chart to identify at what temp the water should boil at based on your elevation and some explanation of why it matters: Boiling Point / Atmospheric Pressure / Altitude
BTW, food cooks more slowly at higher elevation with the same cooker temp...been there...have the slightly embarrassing moments forever engraved in my memory.
As for the water pan, you can use water, washed sand or pea-gravel. If using a solid media, I prefer to fill about 1/2 to 2/3 of level full, then place a couple of layers of heavy-duty foil over it, form to the pan, then wrap tightly over the edges. This will help any vertical smoker in balancing out chamber temp peaks/valleys...though it will not reduce prolonged high temps like water does (water cools through evaporation from the pan, but causes higher fuel use during hot-smoking as a result). I refined my wet-to-dry smoke chamber method in my SV-24...the 24 made it easy to perfect this method.
One word of caution on the chrome-plated cooking grates: do not use abrasives or scouring pads to clean them...the finish will rub off in no time at all, and you will be left with, first, a copper-clad (which oxides the meats, turning them green in the grate marks), then, bare metal wire, which will corrode if not coated with oil. Best option is to brush or spray with cooking oil, season the smoker as normal, then, re-apply cooking oil before each use. Let the residue build-up and form a petina on the grates...this will become a non-stick finish, given enough time, and will protect the grates from damage as well. I did not do all of this with mine until it was too late...I can probably salvage the grates if I work on the petina long enough, but I'm currently in the market for stainless steel grates, which they don't make...so I may end up getting stainless expanded sheets to make new grates...I would prefer stainless any day, as you really can't damage it, unless maybe bt cleaning it with a carbon-steel brush...not sure on that.
My SV-24 quickly became my main smoker (even with my SnP 40" pit burning charcoal or running on my gas conversion), from smaller meals to larger gatherings...it does it's job very well...enjoy your Vault!!!
Don't be afraid to experiment with smoked potatoes, cabbage, squash, garlic, onions, mushrooms, peppers/chilies...whatever blows your smoke rings...LOL!!!