1. Stan's is the most popular sealant for tubeless. It is latex based and will dry out. How often you need to refresh it wl depend on weather. I would ask the locals what they use.
Tubeless can be frustrating if you are not using tubeless compatible rims and tires.
2. Buying a great bike is always better. But it will depend on your budget.
3. The hardest/most expensive parts to upgrade will be frame, fork, and wheels.