5550 is maybe a bit too much IMO. I would recommend a second hand H3970...has a 400MHZ CPU and Bluetooth built in. It wont take USB HDDs though (dont think any PDAs do) but can take up to 1GB SD card and should work flawlessly with your Navman sleeve. We sometimes get refurbished stock in if you keep an eye on
http://www.totalpda.co.uk/section317.aspx but we dont have any right now. Check ebay.