1. I use my equipment interchangeably, and it's worked out well.
2. I just collect empty wine bottles, so I pay nothing for them. You can ask bars, restaurants, friends, etc, or drink a lot of wine, but free bottles are my favorite kind.
3. I always follow the kit directions. Sure, making 5 gallons out of a 6 gallon kit might give more body, but the flavor and acid adjustments and the gravity (and hence alcohol level) would be askew and the wine wouldn't be balanced. All the wine kits I"ve ever done (+/- 100) don't have an issue with body at all.