Dave, some fit front dynamo hubs - they are quite efficient. 3 to 6 watts. SON is one brand. Loved by longer distance touring bicycle riders.

chilswelluk -
You want a Garmin touring model .. don't worry about any cadence etc fittings - just don't use them. THe 'Edge' series ... I like a colour screen to aid map display .. B&W are better for battery life but it needs to be usable too.

Glonass support - improves accuracy and start up times. If it has it fine, if it does not also fine.

World map .. these have little detail and are useless when your trying to navigate in a city or out in the country, only usefull for a broad plan. Just use the county maps. If someone made a world map with all the detail .. the size would be huge - probably slowing the thing down, and making updates over wifi take days to download. Forget a world map.

You will need a way of changing the GPS map .. depends on what model you get - the worst ones will require a USB connection to something that can transfer maps. A smart phone with USB OTG will do that. The better ones use a memory card - you can then put different maps on different cards. Newer models have the ability to have different maps on the same card and can swap between what maps they use.

You will find a smart phone is very usefull .. get a cheap one. It has a bigger screen that your GPS, useful for phone, alarm clock, camera, wifi and another map to use with different routing that may be better to use than the GPS.

Back roads/paths will depend on what is in the map you have, OSM is fine is some places and lass so in others ... usually where OSM lacks this road detail .. so too do the commercial maps simply due to lack of demand.

Hope that helps.