Recently, I wrote a (too long) summary of pretty much everything any photographer would need to know about legacy flash units using thyristor technology like the old Vivitar, Metz, and Sunpak flashes.
This article is only peripherally related to Leica but there is mention of the Leica SF24D and SF20. That article is super long but if you are only interested in the Leica flashes, here is the summary. Get one. You dont need TTL in your camera to make them work. That is the beauty of the thyristor technology and, for whatever reason, Leica included it in those flashes. They are tiny. They are also cheap.
If those are too small and you need some more juice in your flash, my next recommendation would be any of the newer Nikon flashes in the SB800 era. The flash voltage wont fry your camera and they are the most fully featured for the price.
I tested alot (way too many) flashes for that article. I dont see any reason to mess around with the older flashes with the higher discharge voltages. I don’t care what you hear about the Vivitar 285.