This guide won't cover the basics of equipment unless requested. The basics are covered on the wargaming wiki.
Equipment is chosen to improve on the qualities of the tank, it is considered better to improve an above average characteristic of the vehicle in order to maximize on its strengths than make its below average stats a bit better without making them excellent.
The equipment is also tied with the skills and perks of the crew.
The best moment is to buy equipment when it is discounted by 50%, which happens once every 1-2 months. If credits are not an issue, then please pass this advice.
The “poor man's setup” is: Toolbox - Camo net - Binoculars.
Please note that Toolbox, Camo net - Binoculars are the only pieces of equipment that can be swapped between tanks for free. There is thus no reason to have more than 1-2 of those, and swap them between your tanks. There are some mods out there that help you to automatically mount/demount this equipment for a major QoL improvement.
Recommended setups for different kind of tanks:
Passive scouts: Improved Ventilation Class X + Camo Net + Binoculars
Active scouts: Improved Ventilation Class X + Gun Rammer + Optics. Vertical Stabilizer should be considered and should replace Improved Ventilation Class X or Rammer when available.
Medium Tanks: Improved Ventilation Class X + Gun Rammer + Optics. Vertical Stabilizer should be considered and should replace Improved Ventilation Class X or Rammer when available. Just very rarely you want a passive setup for your medium tank, only when you are still inexperienced in flanking, wolfpacking, etc.
Heavy Tanks: Gun Rammer is in most cases a must due to the very slow rate of fire of heavies. The other 2 can be chosen according to the stats of the gun: Improved Ventilation Class 3 on better guns and Enhanced Gun Laying Drive and Vertical Stabilizer on poorer guns.
Tank Destroyers: Gun Rammer is in most cases a must due to the very slow rate of fire of tank destroyers. The other 2 can be chosen according to the stats of the gun: either Improved Ventilation Class 3 and Enhanced Gun Laying Drive or Camo Net and Binoculars can be chosen if you have a very poor armoured TD and the gun stats don't hurt you. The last 2 can be swapped as preferred.
Artilery: Gun Rammer and Enhanced Gun Laying Drive are a must, Improved Ventilation for the closed ones. For open ones, neither the Toolbox nor the Camo Net won't save you from 1-2 shots.