Be aware that copper alloys (brass etc) are susceptible (more than steel) to fatigue cracking due to work hardening. If you use copper, make sure to support the various tubing runs and branches with brackets. Steel is stronger, readily available, works well, and is easy to silver braze. The melt temperature of the braze alloy is a function of the silver content. More silver means a lower melt temperature. More silver is also what makes silver brazing easy to do. It's a compromise.

Good luck.