Conversion to a Canadian PPL is an unnecessary intermediate step unless you need (or want!) to do hour building and an FLVC is unsuitable. You can go directly to a CPL course with a non-Canadian PPL. The FLVC allows you to act as PIC on the strength of your UK PPL and UK medical certificate. You can expect to wait at least a couple of months, perhaps half a year, for a Canadian medical certificate to be issued so don't delay the assessment. You'll also need the Canadian medical to write exams!
Two caveats on hour building. It is unclear whether 30 hours of solo flight time, part of the flight experience specified in the CARS for the CPL, must be done under the supervision of a (Canadian) flight instructor. See discussion
here. Second, "solo flight time
means, with respect to the flight time necessary to acquire a permit, licence or rating ... in the case of a pilot, the flight time during which the pilot is the sole flight crew member ..." CAR
400.01(1).
A PGWP is available only for approved courses, lasting at least 8 months, done at a DLI. See summary of rules
here. The permit is issued for a period equal to the time spent studying. If you study for two years, it may be possible to apply for a three-year permit, which is the maximum period given. You can stack courses together if needed. There is a lifetime limit of one PGWP per person. Most western nationals can pursue flight training in Canada up to six months as a visitor, without a study permit.
AFAIK, a study permit requires the courseload to be at least 15 hours per week. The study permit should allow you to work, on- or off-campus, for up to 24 hours per week, assuming you're not competing with half of Punjab, eg
here. If you're aiming to instruct, you might pick up a low-skilled job at the FTU.
Why do you want 1 500 hours specifically? Are you mixing up US and Canadian industry requirements? The total flight experience for a Canadian CPL is 200 hours, not 250 hours. Also you don't need a group 1 (multiengine) instrument rating to instruct. Canada is coming out of a bubble in flight training right now so the demand for instructors isn't as acute as last summer.