I think the most cost-effective solution is the K&K audio TOTL phono stage, balanced version. For a bit extra you can have it with adjustable loading, and in my opinion, balanced phono is an advantage as well, for around $3K or less, in factory-assembled form, I think. Talk to Kevin Carter about your needs, and he can make it happen. (No affiliation.)
Further, if there's sufficient room on and in the chassis, it's no big deal to install adjustable loading for R and C yourself. The trade-off is said to be in terms of noise; the switch required for adjustment is going to introduce a very tiny amount of noise. I don't hear it even on my very high gain MC phono stage, so it really should not be a problem at the lower gain levels needed for MM.
Further, if there's sufficient room on and in the chassis, it's no big deal to install adjustable loading for R and C yourself. The trade-off is said to be in terms of noise; the switch required for adjustment is going to introduce a very tiny amount of noise. I don't hear it even on my very high gain MC phono stage, so it really should not be a problem at the lower gain levels needed for MM.