You pretty much get what you pay for with satnavs. I'd stick with Garmin or TomTom.
The TomTom One (UK and Ireland) can be had for around the £100 mark. It will navigate you as well as any of the more expensive models, but doesn't have any of the bells and whistles.
Got one myself, never let me down.
Oh, and it runs Linux and there is an active community of native app developers for it. There is even an X11-Basic interpreter for it.
IIRC, the Garmin devices use a proprietary OS, not Linux.