depending on what you want to do with it depends on what guage steel. lower the number thicker the steel. in sca helms are 12/14 guage and elbows/knees are 16. soem steel fighting groups everything needs to be 16 guage min. etc. for just looking cool 18 and higher will work great. my stuff is a mix of 18 and 16 guage.
Agreed, purpose of the armor has a lot of bearing over what to choose.
The type of steel also has an influence. The "normal" armor steel could be 18 gauge for a non-combat kit, but I would not go lower than 14 gauge for combat; possibly 12 gauge if it has a domed section. Some helms will be constructed of 2 different gauges, since the dishing thins the metal in parts.
There is also "case hardened" armor. The hardening increases it's ability to take punishment, so you can go "down" a gauge. Such as hardened 16 gauge helms will perform roughly similar to a 14 gauge unhardened helm. Only certain types of steel can be hardened, though.
Stainless steel is also more dent resistant than mild, low carbon steel, and the same general rule applies as for case hardened armor. Late period armor was case hardened, but (to my knowledge) no such thing as stainless steel exists in period pieces.
There are also "mix and match" armor styles; thickness of metal on a historical suit varied widely. So in one harness, you may have a 12 gauge helm top, 14 gauge helm sides, a 14 gauge breastplate, 16 gauge articulation lames, 14 gauge knee/elbow cops, and an 18 gauge backplate and greaves. The armor would be thicker in areas likely to be hit, and thinner in less vulnerable areas to help with weight and mobility.
If it's armor JUST for a kit and won't see full-contact combat, I would suggest 16/18 gauge (16 gauge cops, 18 gauge lames/etc), since it will probably be worn for much longer times (such as all day at a faire) and will be much, much more comfortable. My 16 gauge mild gothic harness is a bit brutal to wear; I'm tired in about 15-20 minutes. I still have some tweaking to try with padding the shoulders a bit more (the straps dig in a bit), but the overall weight is quite heavy for a middle-aged chubby desk monkey such as myself.