For alfalfa or alfalfa mixed hay I would look for a belt baler. The chain/bar balers beat a lot of the leaves off the hay as they baled.
I have had pretty good luck with the Vermeer 504 balers. I think they where open throat after the "F" series. They will be some one here that can tell you for sure. They would be fairly reasonable in price and parts are readily available.
A JD 435 baler would work too but they would be a little higher in price and the tie setup is a little more complicated if your not familiar with them.