Your chance of challenging the tests for your journeyman's is largely dependant on which province you are in. Some provinces (Alberta in particular) has a high regard for our training and you are usually able to challenge after a certain amount of hours (sorry, can't remember how many).
Other provinces (Quebec and Manitoba in particular) make things a bit more difficult (have to have some schooling done in those provinces or been a licensed tech in another province). My info is a bit dated, been out of the trade for a few years now so things may have changed with the various provincial apprentice boards.
There are benefits of getting training first and then joining. You will get to skip some parts of your DP1/DP2 training, but you will still have to do the military type components (Regimental Company being probably the most important where you learn your tactical drills and just what a Mobile Repair Team does) such as tracked vehicles and recovery section.
As for work, your initial posting would likely be to a 2nd Line unit (Service Battalion) where you will do jobs on major components (engines, trannys, differentials). Basically, 2nd line work is any job that takes over 4 hours to do (it's way more complicated than that in practice, but that's the basic theory). After you do that for a few years and complete your Developmental Period 2 course (what we used to call QL5), you may get posted to a 1st line unit.
1st Line (field units or operational airbases) is the quick and dirty stuff, repairs that can be done in-situ (where it breaks) and in under a couple hours (tires, brakes, lights, alternators, engine parts, but not a whole engine).
In either type of unit, you can end up working on everything from small engines/components (augers, generators, snowmobiles), B vehicles (trucks of all sizes and shapes), A vehicles (tanks, LAV III, TLAV) and recovery (tow trucks).
