the SB 400 / 600 only work on Nikons with TTL flash metering, as they don't come with their own light metering cell. Of course you ca use the 600 totally manually, but given your intro I don't think that makes sense.
I've just bought a SB22 from Popflash for US 25, as it's a tad smaller than my SB 27 and still uses AA cells, which I prefer. Some Nikon flashes known to work (as they do have non-TTL metering):
SB 800, SB 80, SB 24/5/6/8, SB 30, SB 27, SB 22(s).
There are tons of others of course. Usually said, if you get a Nikon / Minolta / Canon flash from the 80's, you can be almost sure it works. With older flashes, there may be an issue with the trigger voltage. Check
here, if you're interested.
Don't get something like a SB 24 form factor flash without trying first on your M6. It's more the camera hanging off the flash. The SB 30 is maybe the smallest one, albeit not using the nice AA cells.
Given your all your cameras, I'd go either for a cheap second hand SB 27, SB 30 or a SB 22s.
If you really use flash a lot, forget the SB 30. If you need lots of power and want the all-singing-all-dancing, my all means get the SB 800. If you do fancy macro work with the D50, get the SB800 as well.
I personally prefer non-TTL flash especially on Digital, since it does NOT have the very nasty pre-flash and delay. That makes choices cheap.