One other set of specs that exceed the power necessary to stop airsoft projectiles are the mil specs complying with
U.S. MIL SPEC MIL-DTL-43511D (Clause 3.5.10)
U.S. MIL-PRF-32432 (Clause 4.4.3.3.5) which supersedes the above spec
Both of those can stop buckshot from a shotgun, an airsoft projectile is nothing to them.
There are many things that you can go cheap in with airsoft and suffer no real penalties, cheap guns can shoot great, cheap gear will work until it breaks but when it comes to eye protection its got to work and fogging is probably the number 1 issue people have and you certainly don't want it to fail and loose an eye.
If you are before your first game then go out and rent for your first few games, you need 3 games under your belt to get UKARA anyway its mostly the cheaper route to rent gear and they will provide eye protection and the basics to play. You'll then be safe and getting a feel for the game but you can spend time in the safe zone asking people about the gear they have, what they like and dislike about it and get a pretty good feel for what is good. If there is one thing airsofters love doing its talking about their gear. Use those few games renting stuff to get a kit list and if you have survived it and still want to play then you'll be able to budget more appropriately to get stuff that is going to work well.