To be blunt, you'll have a real hard time cracking the top 5 in the national series and winning the OPRC with a $25,000 car.  It can be done, but would take extraordinary talent and a lot of experience.  The sharp end of the national field (not even including the Subaru Canada team) spend a ton of money every season.

The purchase price of the car is the proverbial tip of the iceberg.  It's the ongoing costs that are usually the budget killers.  For $20-25K, you can build a nice P3 Golf or P4 2.5RS and run it for a couple of seasons if you don't break too much.  Both cars are competitive in the right hands, so both would give you a shot at class championships.

There are some good drivers in these classes, so don't expect to win right off the bat.  Start out by aiming for finishes and fun, attend a rally driving school, and get stage miles under your belt.