The best time to knock is when they're in :

Not meaning to be sarcastic - but I think any time (other than in the middle of the night, first thing in the morning) is ok if you can get a face to face with them.
As to employing canvassers - you can pay them how you like - it's a straight forward agreement between you. It will only be part time work so you should be ok for them just to invoice you for the agreed rate (150-200% of the job single visit value, for instance)
My feeling is that unless you need vast quantities of leads, the best person to do the canvassing is you - you will come across as much more dedicated and genuine and nobody knows your business better than you do. Plus you won't be disheartened any where near so easily as a commission paid worker.