The Weber Smokey Mountain (WBS) is known for its active community and the plethora of mods and accessories. I’d say the only mandatory thing you really need to do to your WBS is to get a proper thermostat as the temperature gauge on the top is very inaccurate.
I opted for a ThermoPro TP20, which has two probes. Whatever you get, I would suggest getting one with at least two probes, since you want to track both the temperature in the smoker, as well as inside the meat. While you can get fancy mounts for the WBS to mount your thermometer, I opted for these cheep magnetic hooks hooks (5 quid for 4) that does the job just fine.
In addition to this, I am also considering attaching a hinge to the lid (there are lot of 3rd party mods) so that the lid can be lifted up without having to put down on the ground.