You might look at dahon's hub dynamo at low price priority,
and Schmidt 20 as a long term investment..

so there is the length of service life thing too ..
I own 2 Schmidt 28 types.

Somewhere I saw a tech manual for a shimano freehub with a dynamo in it.
I deal for tadpole trikes , but also good on recumbents as the rear wheel
If that works better..