Although I've never used any other devices with rockbox, imo the ipod6g/classic is the best.
You won't find one new, but if you search ebay with this query:
ipod -video -touch -2nd -3rd -4th -5th -shuffle -nano -mini -8gb -4gb
I search that way because sometimes someone will find a classic and sell it without classic in the title not knowing the model.
You'll start to see some 80GB ipods or other models that can be bought for $50 with working mechanical HDDs and no need to replace with flash.