What you should know about buying armor. Class > Type > Rank.
FOR EXAMPLE:
Armor Type is the key here.
Bronze Cuirass gives me more defense than anything rank-appropriate at Rank19 Gladiator. This is because the Cuirass type is designed to give the most defense.
Class is also very important.
The Dodoskin Jerkin gives me (at least) half the stats of the Cuirass because gladiator is not listed on the item. This is why Weavers should not wear full doublets while crafting, only doublet vests, because weaver is not listed on the full doublet.
Rank is not as important as you would think when it comes to armor.
I bought an Iron Celata to replace my Bronze Celata. The two items gave me the same exact stats. I also own Hempen, Cotton, and Canvas Kecks. All three items give me the same exact stats (Edit: I tested kecks before I was rank19 for the cotton derivative).
As far as shields go, I always use Scutum type shields because they do not fail during battle. I've used Hoplons, and too often for my taste, the message "Your block fails" would show up in log. Scutums never fail, ever.
But, when it comes to Weapons, though, it's important to get a rank-appropriate weapon. I get more accuracy and attack from a Brass Dagger at 19 than an Iron Shortsword.
In conclusion, while you may not get the maximum stats from higher-rank gear, if you have the money to blow, you can't go wrong with buying higher-ranked armor - so long as the gear is class-appropriate.