I took the cheap route: 26 gauge snaplock pipe from HVACommerce.com
($5.66 for 5" x 5'), "economy" 26 gauge fittings from Penn State
Industries, and I installed it myself. The snaplock really isn't hard
to work with - there's a bit of a trick to snapping it, but you'll get
the hang of it after a few pieces. Definitely not a $95/hr skill level.
As far as attaching the flex hose, you'll amost certainly want a blast
gate at each tool, so the hose will usually connect to the gate, not
directly to the pipe.
$400 for design? Humph. This ain't rocket science, and you don't need
CAD drawings with 0.001" precision. Spend $20 on a decent book, and you
can do the pressure loss, airflow, and air velocity calculations
yourself. If you don't feel like doing the math, it's a fairly safe bet
that 5" pipe is the right size for an average hobby shop, using one
machine at a time.