A gauge for coolant pressure should have full scale reading of at least
20-25 psi since you're expecting readings up to 15 psi or so.  Ideally it
should also not be pinned at zero, but able to register negative pressures
of a psi or two.

If you don't want coolant to be forced into the gauge, use a length of hose
between cooling system and gauge, and keep the gauge and hose above system
level.

Most Bourdon tube-type gauge mechanisms are soldered brass and wouldn't be
bothered by coolant.  For digital gauges with silicon sensors, I suspect
that resistance to liquids might vary widely but I have no actual knowledge.

Yrs,
d