I wouldn't count on 5W getting you a whole lot of packets from remote locations to digipeaters. But honestly the bigger issue would be location rather than power. IOW, 5W from a hill top is ten times better than 500W in a valley. So I guess my response is really more like that would be fine.
What you're proposing is probably the second most common APRS type installation and the most common dumb beacon: cheap Chinese radio + small TNC.
Realize that the AvMap is very expensive and only fully useful with a Kenwood TM-D700/710 or the TH-D72. It'll work with any regular TNC to display waypoints but the fancy stuff like the messaging extend the Kenwood specific protocol. With other radios most of the functionality doesn't extend, mainly the ability to receive and send messages on it. So it's really just a very expensive display for APRS stations in that case.
IMHO I'd avoid unless you have a Kenwood radio. You can duplicate the functions using APRSdroid with a tablet and the Tinytrak4 for a lot less cash. Nice thing about going this route is you can link the TinyTrak and the tablet via Bluetooth so there's less mess of cabling. In fact you can rig up the radio to talk to a tablet using the mic and headphones and not even need a dedicated TNC to run APRSdroid.
Truth is a tablet (or maybe you already have an Android phone) isn't much more than a TinyTrak. APRSdroid leverages the built-in GPS of the device, so no extra stuff hanging around.
But I have a TinyTrak4 and it's a handy little bugger. I have a Garmin GPS 18 puck I run with it and it's reliable and works well for a beacon with any radio. I have a little hacked display that shows raw packets and can interface a PS/2 keyboard, so it can do messages. It's not very portable, no case and trying to deal with a keyboard is a PITA. But it's fun to mess with it.