am i missing anything vital for stable performance?
is there a better way to protect from rev voltage?
do i need capacitors for smoothing?
As G_M noted, you have to have bypass [typically small ceramic] and reservoir
[typically largish electrolytic] capacitors on your pcb for filtering and stability.
Also, in regards "is there a better way to protect from rev voltage?", once you
have the diode installed in the Vin line, a MUCH MUCH bigger worry is protection
for all the I/O pins on the Arduino.
Your field guys will likely [with non-trivial probability] be miswiring 12V straight into
the 5V I/O pins with regularity, so you should think about protection on "all" i/O
pins. Typically, a small series-R of about 220-330 ohms will go a long way towards
saving the Arduino's little butt, but for extreme environments, you'll need even more
protection. See ruggedcircuits.com [I think].