agree... being able to buy everything would make no point to it at all - you could simply unlock upgraded units right away as well. however, this is not the point of this thread, which seems to be making all the "unlock upgrades" achieves without too much effort and playing campaign over again. personally i did them on my first run through casual campaign, just saving all the money till it was time to go on char. saved, bought upgrades, loaded, bought different upgrades etc. did the same with mercs, after final load i went for the upgrades i wanted and headed for the last missions.
upgrades during the game wasnt needed at all, tbh, they shouldnt be needed that bad on higher difficulties as well, considering you use many units only in the mission you get them (when you cant have upgrades anyway) and they are little to no use in any following mission. firebats, hellions, vultures, goliaths, diamondbacks, reapers, vikings, thors, BCs and ghosts being a list of them, depending on your gameplay you could add some more to it. so - upgrade your marines, bunkers, terran buildings and banshees when you get them, the rest of the money you can save for the achieves. if you plan to do the last misson with disabling nydus, you can skip banshees too