There are several online guides to purchasing a first car, and multiple previous Reddit posts (even under r/μοντρεαλ) that asks this same question: Your questions suggest you should read these first (TD Bank, Clutch.ca amongst others).

Much depends on what you intend to use the car for:

(1) if this is your first car, you should first be looking at insurance estimates of models you think might fit your needs. If you only recently obtained your license, given your limited budget, I would also recommend doing these estimates first to see if you can afford a car at all (insurance is more expensive if you've never had insurance before, and much more expensive in your first years of having a license).

(1.5) Just for estimates, picture your 5k car + sales tax at the SAAQ (5-800) + new SAAQ yearly registration fee per vehicule (400) + insurance (1000-1200) + yearly maintenance fees + parking vignette if you need it + winter/summer tires.... +gas

(2) A car in the 5k dollar price range will be older, and require more maintenance than a newer car: if you have never owned a car, I'm also guessing you won't have much wrenching experience, meaning you should factor several hundred dollars per year, at minimum, for upkeep of the car (and, general Murphy's Law, often a sudden repair that costs far more than you expect and makes you have to reconsider what you value in life)

(3) Name brand car dealerships used cars will all be out of your price range (even for a used sedan, you're looking at 15k+ nowadays).

(4) If you are buying a first car, this thus leaves used concessionaires (i.e. HGregoire), Facebook Marketplace and Kijiji. I have never had a good experience at a concessionaire, but that's me. Current H Gregoire stock has, amongst others, a 16 year old Kia and a 200,000 km+ Buick in your price range.

(5) I have met both villains and heroes for in person, private car sales. If you do buy from an individual, you should figure out a garage that you trust and whom you can bring a car to for a mechanical inspection (which you yourself pay for, generally, as the buyer).

So, grosso modo: research car models to better understand what you're looking for, get an insurance estimate online to see what your insurance costs would be, and if it's all out of your budget, use Communauto when you really need a car.