I'd recommend one of two options:
- shop around and get a basic monochrome GPS secondhand on
eBay (personally I'd recommend a Garmin eTrex or eTrex Legend). Remember that you will need to get a cable and probably a serial/USB adapter;
- get something like a Garmin eTrex HCx Legend. They cost about £130 from Amazon
I wouldn't recommend getting a new monochrome GPS - mainly because compared with the newer colour models they are pretty poor value. If you find you get on with the GPS, you'll probably find the basic GPSes a bit limiting. If you get a secondhand one you can probably sell it on for most of what you pay for it while if you get a new one you'll take a financial hit.
If you want a GPS for navigation on tour then the HCx is a much better bet - for example it has much more memory for track and routepoints, built in mapping, and a memory card for storing Points of Interest. The Edge 205 is designed as a training aid for people doing day rides. so far as I know it won't even display a grid reference - this seems to me to be something that's absolutely essential. It retails on Amazon for about the same price as the HCx so unless you want the heart rate monitor it's very poor value in comparison.
I also think it's worth paying extra for a GPS with inbuilt mapping - the included mapping doesn't show you all the back lanes and minor roads, but it's pretty useful if it all goes Pete Tong.