For lower cost, get one of the Shimano "A" (Altus, Acera etc.) RDER's in 8 or 9 speed.
If you plan on "upgrading" to index shifting, I'd stick with the 9.
I can see where you might want more of a "road" looking RDER to better fit with the bike, but the above mentioned RDER's are designed for your type of gear range on the cassette.
They shift my "road" cassettes fine too.