Welcome aboard! Tackling a brisket right off the bat was braver than I...lol Now that you've found SMF, you've seen there is a ton of great information and folks here that definitely help us rookies out and shorten the learning curve greatly!
A lot of folks use probes from Wally World or from where ever. The key is to test them, no matter if it cost $20 or $100, for accuracy with boiling water or ice water...even some of the more expensive units have issues. If you have a Lowes store nearby I know I picked up a couple of the Taylor Weekend Warriors for the cost of what you'd pay for one from Wally World. Lowes also (at least here) has the next higher Taylor model with a little alarm pager on clearance for $19.95 (I haven't ever used the pager device though).
After using both Taylor models for a few weekends I decided I wanted a device with a remote unit which landed me now with a couple of the Maverick ET-73s that can be found in the $40 range. Comes with two probes, 1 intended for the meat and 1 intended to monitor chamber temps (but can be used for meat also), has low and high temp alarms for both probes which is great for catching a nap during those all night smokes.
IMHO, you definitely need to get a good digital probe to monitor internal meat temps and you'll want to get one to monitor your chamber temps as you've already read most door gauges are not very accurate. If you can, remove yours and calibrate it...boiling water is 212*.
That was long winded enough, I'm sure others will come along and weigh in shortly. Good luck and again welcome! Happy Smokin'!