If you use as many materials as your class allows, you won't be able to use anymore. The use option won't appear when you try to use a material from your inventory. On this server, the max stats for all the classes have been increased by 100, except for ata, luck and evp. Ata gets 10 points higher, luck doesn't gain any and evp gains +30. If you plan on getting your characters to level 200 you should generally be careful about what materials you use on them, but it's always fine to use HP and TP mats. Generally EVP and DEF are bad stats and not worth maxing so those materials should never be used. The mag you raise isn't too big of a deal since you can always make another when you get to higher levels. God/Units are extremely outclassed by Centurion/Units on this server. You should never use God/Units to max your character out. Generally you want to max out your characters stats while leaving as many slots free for helpful units like cure/units and v502, smartlink etc.
The best slot items for maxing out imo are:
Centurion/Arms: It gives +35 ata which is the equivalent of 70 points on a mag Adept: It gives 20 to ata 10 to all other stats and 6 to all resistences. Which is the equivalent of 55 points on a mag (not including defense or evasion because they aren't useful) The 6 points of resistance do come in handy, it also reduces the TP costs by 1/4.
Centurion/Ability: It gives 3 ata and +30 to all other stats. Which is the equivalent 51 points on a mag.
You generally always want to have a slot opened for units that increase battle speed. Centurion/battle is the best giving a 110% boost. V101 is a much easier to get option, that is still useful. If your character casts techniques often you'll want v801 which also gives +5 mst, which you should keep in mind for maxing. Smartlink is great for the accuracy boosts with guns, and V502 is helpful for weapons with hell (among others) You can find the base stats and max stats of each class at level 200 on a bunch of websites. Your goal is to take the max and subtract the base to see how much of each stat you need to max out, then use materials, your mag, your armor and shield, and units (if needed) to reach those. Red ring boosts ATA over max and gives 20 to all other stats (not including def or evp again) so keep that in mind when maxing. Certain armors can raise stats over max like Sue's coat, Kroes Sweater, Godric Cloak etc, only certain classes can equip these armors.