I have a 12' Redfish Angler and would like to get a depth gage. I was recommended to mount it toward the front of the Yak and Hummingbird brand recommended.

I don't know squat about depth finders, except the Hummingbird is suppose to show fish instead of blips.

Any advice, recommendation on brand and installation (I assume straight forward -- put sensor on inside of hull and batteries inside as well...

Lot's of differant methods for mounting the puck. The Texas Kayak Fisherman site has plenty of examples with pics. The important things to remember are. mount it on the center line not so far forward that there is a air gap when your kayak rocks back on a wave, and make sure there are no air bubbles in you adhesive.

Get the plastic thru-hull transducer and you don't have to jack with mounting the puck, which is a beating and comes un-stuck often. This one is threaded, and can go through one of the scupper holes. It cost me an extra $75 for my piranhamax 210, but I've been told if you buy a humminbird depth finder, and contact them, they'll let you upgrade to that transducer for $25 with the trade in of the transducer that comes with it.