hub dynamo question

I am toying with the idea of getting a hub dynamo for the coming winter,
however the wheels on my road bike only have 20 spokes which is less than
more conventional wheels which I understand have 32 or 36 spokes. Are there
any hub dynamos that would be compatible with my wheels?

Also, how easy would it be to install the dynamo myself, given that the
spokes go straight out from the hub to the rim? It looks as though it should
be easier than rebuilding a conventional wheel but there may be something I
have missed (I have never tried wheelbuilding before).

Adam