Having had exactly the same thought about 18 months ago I bought a 1940 IPAQ from e bay ( which has bluetooth ) for around £70.
The bluetooth receiver unit from tom tom was around £100 and the tom tom mount was around £40. I bought the original software..
It has proved an invaluable tool ( especially around London ) and makes long journeys on unfamiliar roads stress free.
The best thing about it is that it will very quickly find a new route if you go of course to avoid traffic congestion.
If anyone has a link for the tom tom 5 software let me know as the tt3 maps are a little outdated.