If he reads this he'll probably have all of them by the end of the week.
I've never used one to create a slurry, so for conversation sake only:
The DMT grit's needs to be multiplied by roughly 2 to 3 to get to an equivalent Japanese grit (JIS). So a 600 grit is something akin to a 1,500 grit stone which is pretty darn fine.
The 325 grit is about 800 grit give or take.
The 1200 grit would be somewhere north of 3,000 grit I reckon.
Just thinking about creating a slurry with a 3,000 grit stone doesn't sound right so I'd have to say go with either the Fine @ 600 grit or the Coarse @ 325 with my preference perhaps being to the Fine. Doesn't take much to create a slurry.