I usually try to buy the stuff for units I use, you won't have enough at the end of the game to buy all of the armory stuff so you have to choose.

Personally I bought all of the base stuff since I found them quite useful.

Then unit stuffs for the units I mostly use (Wraiths, Battleships, tanks etc.)

I have not tried but I do not think you can earn the credits again, would be a bit easy to spam like the first mission over and over in 10 minutes and buy everything from the beginning.
Kinda defeat the purpose

I agree that the Siege Tanks ones are quite useful in the later games
Quite powerful.

Marauders concussions grenades are very good and helps a lot until the end of the game, marauders were quite a big portion of my defensive line up to the very last mission.

I also took the bunker turrets too, helps a bit.

Medic are good at beginning but I found myself not using them much at the end of the campaign, most of times I use marines/marauders in bunkers only so medic pretty useless at the end.
I am redoing the campaign in Hard now so I might change my credit allocation a bit more

Well, thing is later in the game I hardly use marines/marauders or any infantry for attack, I use ships or vehicules.
infantry are good but to have the equivalent of attack power vehicules give you, you would need lots of them which then become a problem since they are all bunched up together.

Depends how you play anyway I guess, which is why this system is interesting