Regarding soft armor, the biggest thing to watch out for; is UV exposure, and chemical breaking down; especially from sweat.
If the armor has not been worn day in, day out, and has been stored flat in a cool dry place, then the kevlar layering is okay to use. But if it has been worn on a daily basis, then treat it as suspect. If it smells like body odor, avoid it.
For hard plates; it seems there are two camps currently fighting over the merits of the two main types of armor... on one hand, there's the AR500 steel armor which is budget friendly somewhat, BUT the thing is that most AR500 armor is heavy for level of protection and the cheapest ones do not have anti-spalling coating on the back, nor are they comfortable... the other camp is firmly entrenched, ceramics of various types. While more expensive, and more often a single-hit affair; they are lighter, and they are not as susceptible to spalling like plain steel armor.
EDIT; spalling is what happens when a metallic projectile hits a steel plate, it transfer energy through the steel and push out a piece of steel on the other side, which if not coated with anti-spall, will certainly hurt the user, possibly even kill, for it is like shrapnel. This is an old phenomenon found with the first steel armor in vehicles and in WW1 on personal armor plates.
Granted with the addition of a Level 2 or Level 3A soft armor behind the steel, it is offset, but the soft armor does not stop any rifle rounds, so there is the thinking that its not really of use when you have side plates plus front/back plates.
As to who to buy from? I can't make a recommendation here, you gotta do your own research to find what works for you
Surplus armor is 50/50 chances of being okay, or worn out. Surplus carriers are fine for the most part, but it depends on how comfortable they are on you. Some carriers are totally fine being worn under clothing, others, not so much.
Then there is the opinion that if lightweight, mobility, and so on are the prime mission profiles, a large chest rig with a simple front plate-only setup could be of use...but this assumes that you wont get shot in the back.