Just my 2 cents based on my work in that market, so take it with a grain of salt:
1. I prefer to use 9/10oz+ (3.6+mm).
2. I would consider instead of dying veg-tan, using tack-quality latigo, bridle, or harness leathers.
3. See above. Best products depend on circumstances. What are you planning to do with them? Just plain leather belt-style collars? Stamping? Tooling? Other stuff?
Please make sure you're using quality hardware, as well. The economy cheapo stuff really has no place on a dog collar, although many still use it. At the end of the day, you're making something made to contain a beloved predator with teeth and a typical, willful regard for their own safety. First and foremost, collars need to be strong and secure.