Mk X does not have costume unlocks, and can be easily skipped (you can probably get more out of the Jem'Hadar Mk XI stuff than the Mk X MACO/Omega).
Mk XI has the Type 1 costume unlock (body suit, chest armor, bracers, boots, gear belt/harness), and may be worth looking into now that a good source of generic Mk XII purple gear is gone (the old STF store).
Mk XII has the Type 2 costume unlock, which has everything the Type 1 has, as well as the shoulder pads and helmet.
If you complete the Optional objectives for Infected, Khitomer, and Cure Elite (both Space and Ground), Mk XII also has a Type 3 costume unlock, which has different armor pieces from the Type 2.
Finally, the Adapted Mk XII set has the Type 4 costume unlock, which again has a full set of different armor pieces.