In terms of battles, are you intended to be an offensive battle mage, or a support mage who can defend themselves a bit?
Magic is very useful in battle, but the limiting factor is mana. Tactically you need to think of yourself as the classic D&D mage, your spells are awesome, but you only have limited casts and you’ll need to think carefully about how you use them.
Unless you use XP to increase your reserves then you get 4 personal mana per day, with spells costing either 1 or 2 mana per cast. There are ways to replenish mana when you’re not in a fight, and there are crystals and potions to use up as consumables, but it’s worth bearing in mind you’ll spend a lot more time not using spells than you will casting.
In terms of weapon choice, it really depends what else you plan to do. Unless you have heroic calls or spells, then a spear is functionally identical to a staff. It’ll do a point of damage and that’s it. (That said, it is a lot easier to find stab safe spears than stab safe staves. If that’s important to you then bear it in mind.)
If you’re only planning on casting spells like heal and restore limb, you don’t need an implement at all. (although it’s useful to get the benefits of crafted items, so you may want a wand anyway) so there’s no disadvantage to using a spear over a staff, other than the fact that you’ve paid an extra 2 xp for the ability to use a slightly pointy stick instead of a magical one.
If you are set on casting spells in battle, then as others have said you’ll need mage armour. If you wear proper armour you won’t be able to cast at all. My current mage wears heavy chain, but that’s because he has no battlefield spells, so he loses nothing from being unable to cast.
Mage armour is by far the easiest to achieve, since it just needs to “look mystical and be clearly not actual armour” If you can sort something for your chest or a hero belt before the event, then a leather circlet and simple vambraces can be picked up very easily at the event. Take a silver/gold fabric pen to put runes on them and off you go. You’d probably want to improve on them later but it’s pretty easy to hit the basic requirements.
My mage previous to this one was a battle mage, and most of his mage armour was cheap “leather” (would barely have counted as light armour) with lots of mystical squiggles added in gold paint. Think it cost about £40 to get the whole thing done (plus whatever the gold pen cost)