If your $15-20K budget is just for the tractor, you can probably exclude anything bigger than about 40-45hp if you want to look at new or slightly used. Slightly used machines hold most of their value very well. I bought a tractor for $21,500, put 215 hours on it in 15 months, and sold it for $19,250...so not a huge difference.
Any of the big brands make good machines these days, and the only good way for you to pick is to get some seat time in as many as possible to figure out what you like, and don't like. Something that annoys me might not bother you at all, and vice-versa, so it really just depends on the individual, and you don't want to buy a machine with a basic characteristic that bothers you (seating position, control placement, etc) since you're probably going to keep it for a long time.
What brands are within a reasonable driving distance from you? Right now the brands that seem to offer the most bang for your buck are Kioti, LS, TYM and probably Yanmar (good quality, but not a lot of dealers it seems). I wouldn't bother with CaseIH/New Holland since they're outsourcing machines in this size, and you can normally buy them from the actual manufacturer for thousands less.
That leaves you with Mahindra, Massey Ferguson, Kubota and Deere as the more expensive brands, with Deere usually being the most expensive. Some of the Masseys and Mahindras in this size are outsourced, but from good companies. You generally can't buy the Massey equivalent from the manufacturer here in the U.S. and I doubt in Canada. I think there is some crossover in the Mahindra line where you can buy the equivalent from TYM, but that may have changed with the newer series machines. Mahindra also has had some of their line made by Mitsubishi, but that's another one you can't normally buy from directly. I don't have a big issue with outsourcing if a company is honest about it, but I'd give an edge to a company that makes their own machine, because they have more control over how long they support it with parts.