I'm looking for a holster for a full size P320 but I know almost nothing about holsters. So I have a few questions.
1. I want to put a TLR1-HL light on the pistol, but don't have one yet. If I buy a holster to fit the light, will it cause any issues to use it without the light?
Some holsters may work with both (light and no light), but most won't.
2. I'd like to buy Canadian if possible, but will take other options under consideration if they're good quality. Suggestions?
Raven, Dark Star, Keepers, J M Custom Kydex, Safariland, Blade-Tech.
3. Primary purpose is just range use to keep my hands free. This is not a duty pistol. Any features I may want or not want?
I suggest that you consider:
primary retention only (meaning no extra straps, buttons, latches, levers, or lanyards);
covered trigger (meaning that the holster protects the trigger from any inadvertant manipulation;
dedicated fitment (meaning no 'will-fit'/'may-fit' holsters);
rigid construction (meaning no nylon, or fabric, or collapsable material); and
an unimpeded draw (meaning nothing that will interfere with a master grip from the holster, and nothing that will interfere with a master grip during the re-holster).
4. Do I need a different belt? I'd like some magazine holders as well.
You might not "need" a different belt, but you'll want a purpose-made belt that is very rigid. If what you want was duty-oriented, you'd be looking at wider belts and accessories, but for carry/range/match use you'll probably want to choose from 1 1/4", 1 1/2", or 1 3/4". (Whichever you choose, you'll need to match belt loops, belt, holster, & pouches.) I think that it's safe to say that for carry/range/match use 1 1/2" is the most commonly-seen width (and it's what I use). You might choose a (two-belt) 'inner-outer belt' system, which is popular for IPSC and some duty use, a 'battle belt', or a (threaded through your belt loops) one-belt system, which is popular for carry, range use, and IDPA.
5. What level of retention would you suggest for range use?
Level I. Some people feel it necessary to have Level II (or Level III, or Level IV), but I strongly suggest that you consider Level I.
Here are three examples of holsters that I'd recommend to you for your P320:
Perun
<https://rcsgear.com/perun-strongside-owb-holster>
Light-compatible Perun
<https://rcsgear.com/perun-lc-holster>
Range Competition OWB Holster
<https://www.jmcustomkydex.com/c/Range-Competition-Holsters.html>
Range Competition OWB Light-Bearing Holster
<https://www.jmcustomkydex.com/c/Range-Compeition-OWB-Light-Bearing-Holster.html>
Classic OWB Holster
<https://blade-tech.com/products/classic-owb-holster>