Solvent does not remove rust.
You degrease the part, then manually remove the rust, then clean with the solvent just before welding. Acetone or alcohol are better cleaners for weld prep than brake cleaner. These solvents are volatile enough that most of the time, the part is dry and not-flammable by the time you get your gear on and are ready to weld.
The really damaging thing here is not the fire but if you use chlorinated brake cleaner when welding it created concentrated chlorine gas and will kill you.
Welding produces a ton of nasty fumes and you should ALWAYS be wearing a welding rated respirator and using a fume extractor when welding, especially with flux core, galvanized, or stainless unless you want extra nasty cancer.
It doesn't sound like you want to be talked into it.
Working with the epoxy is messy and they can be hazardous but the finishing work is the really nasty part. Doing any grinding or sanding on composites is MISERABLE. The powder created gets everywhere, is itchy, and will give you cancer if you breathe it.
Are you planning an electric or acoustic?
Integrated neck, bolt on, or set neck? Wooden or carbon fiber? What fretboard material?
Have you built a guitar before?
Have you worked with epoxy/composites?