If you take a look at
http://www.globalpositioningsystems.co.uk they have some good deals on TomTom with PDAs and non-TomTom GPS receivers (ofter referred to as GPS mice)... I'd recommend going for a 400Mhz PDA, speeds up the route plotting a bit, and try to use SD memory to hold your maps as it's a little faster than CF.
If you have the heated windscreen, it may have a coating that prevents the GPS from getting a good signal.
One thing you might like to look at for TomToms is CheckPOInt - free software that has a database of speed cameras in the UK... works very well.
I haven't found mounting too much of a problem, I got a flexible suction mount from the site above and that works well - I have it sited to the right of the steering wheel above the vent. Other solutions include the Brodit Pro-clip (popular right now for iPod mounting) but I find the positioning then too low.
