I have the Optometrics 300/ 600/1200/ 2400 gratings; For a multipurpose littrow I'd start with the 600lpm, it gives a brigher spectrum and enough dispersion to give you as much detail as you'll need.
I have the 600lpm in the Classic with a 160mm collimator and a 135mm camera lens and went for the 1200lpm in the 200mm littrow.
The 600lpm is easier to use!
remember you can always "upgrade" later!
Just remember you need to watch the width of the slit! Depending on the fl of the collimator this will be somewhere between 25 and 35 micron..
I would agree on a 600 grating for a LHIRES III type clone. I had 150 and 2400 l/mm originally but now have a 600 that I wish I had bought first as it is a good balance between resolution and ease of use to get started with. (don't forget to allow enough length in the grating/holder to cover the beam when the grating is at a shallow angle. With the LHIRES at f10 you can get away with a 25mm square grating at 600 l/mm and below but at 2400 (and I believe 1200) it needs a 25x50mm grating
