1) I'd say it'll safely fit 42mm tires. Not sure above that but at least 42 with fenders.

2) rear tubus cargo is probably the best rack on the market all things considered. Not too pricey and Tubus has a crapload of spare parts / mounting options available. Steel cromo construction and the cargo has 80lbs weight limit.
Up front a Tubus Tara. Simple but effective
Also the thing to consider about tubus is that they aren't heavy at all.

3) fender mounting is dead easy. Absolutely no issue. Also the tubus tara you'll be getting has additional fender mounting points

4)i have a front basket in my commuter setting so I store my locks there. Some people use bags / backpacks etc. Your choice

5)Ortlieb ultimate6 is really nice. It locks the lid on and locks onto the bike and is completely waterproof

6)panniers are expensive. Don't use them if you don't need them. But they do allow for a car free lifestyle ss you can transport your stuff in them (groceries, work clothes etc)