You can buy weatherproof WinCE units very cheap from eBay, see auctions 232799360784 (4.3") and 172004518145 (5") for examples. There are many sellers selling these same units at many different prices but they all seem to be identical. I have used both sizes and they work well although the screens are a bit faint in bright sunlight. Amusingly enough one of the units I bought came with a sys.txt file signed by one of this forum's contributors!

Next I recommend you install the programme from this thread, its fast and stable.
Spoiler: Civic Reborn
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The maps supplied with the unit will be well out of date so download and install your chosen maps from the appropriate section of this site and you are good to go.

I have travelled many thousands of km around Europe relying on these devices with no problems, even in heavy rain.