I do have experience comparing the SL to the ML. For me, the difference was very much dependent on the table. I used the ML on three different tables and found that it sounded notably better only on one of them. The SL Mk 1 has been my go-to for several years now.
If I were in a replacement situation like yours, I’d probably buy a SH Mk 1 on the clearance sale. The Darlington does have a LOMC setting, but you’d likely get better overall performance with the high output cartridge. I did not seek much advice on choosing a SL versus the SH, but a high-output MC can be a better route unless you want to slide down the phono/SUT/SU upgrade rabbit hole.
What’s the benefit of using the SU-7 with the MP8B?
Do you have a low-output MC cart? If not, there is no benefit and, as I understand it, the SU-7 would make no sense and be incompatible unless you’re using a LOMC. Darlington’s site has a lot of info in the FAQ and Support sections. Here is a small blurb about your question, but I think there’s a bunch more if you dig around, although I’m not certain of that bc, while I have the MP8B, I use high output MC cart primarily and my spare is MM. Lastly, if you think you want a SU-7 for whatever reasons, they’re about to release a new...