The Starcraft 2 Campaign Mission Order question has already asked
Will picking missions randomly from
the planets available exclude me from
doing other missions or will it all
just sort itself out?
I am curious what is the most efficient mission order in regard to unit upgrades because whatever unit upgrades at the time of the first play through in a campaign save so what you are stuck with when you replay that mission. Unit upgrades should benefit achievement hunting. Both research and unit purchase upgrades should be considered.
The Evacuation mission, for example, you should upgraded your bunkers before taking on this mission using tier I zerg research to assist in the Sacrifice Nothing achievement.
Best Answer
Once your Zerg/Protoss research is capped out at 25/25, replaying old missions will grant you 10k credits for each research point you re-earn. This means that as soon as you progress far enough through the campaign to max out your research, you can take a break from the new missions to grind credits for any unit/building upgrades you may have missed.
This cuts a lot of variables out of an optimal achievement-hunting run since you only have to have a plan up to the point in the campaign where you cap your research points, but there are still a lot of things to consider. I'm assuming that you aren't as concerned about what order units are unlocked in, since that depends on a lot of things and it's hard to provide a definitive answer.
Another variable to consider is that achievements aren't tied to a particular save. You can always save, spend your research/credits on one set of upgrades, play a mission to unlock the achievements, then reload your save to spend the credits/points elsewhere.