Providing you can plug your GPS into the a/c power, I'd recommend getting a Windows CE powered GPS device from Ebay. You can pick up the Aware unit for somewhere in the region of £30-£50, but there are lots of alternatives on there.
Then, budget dependant, you have some choice over what you install on it. If you just want a map then Memory Map will suffice. But for planning and plotting routes you could go for SkyDemon Mobile which lets you plan on your PC and then transfer that onto your GPS. You can also create routes on the fly using your GPS.