From the relevant page, same as those weapons sizes:
"Armour protects the wearer by increasing their global hits if it covers the majority of the torso and at least one other location. Valid locations are the head (with a helm), the arms and the legs, or you may cover half of your arms and legs.
E.g. A mail vest covers your torso but does not cover another location. If you wear it with a helm them it would count."
For a suit of heavy armour, you’ll need full torso coverage (front and back) and a helm. Not many of the orcs wear a metal helmet, as it’s extra wieght and encumberance over a full head mask.
The other option is full torso, greaves and bracers (ie, half arms and half legs = 1 extra location, + torso = a set).
This is popular because a full metal helm (or even a chainmail coif) is pricey compared to greaves and bracers (I can reccomend White Rose Apperel for good value plate here).
Mechanically, heavy armour stops CLEAVE and IMPALE shots. If you’re carrying a shield, covering you between shoulder and knee, some smug sniper shoots you in the shin, that’s an IMPALE to the leg, and you fall over. There’s plenty of happy barbarian orcs who will go for an unprotected leg with a CLEAVE call, for the same result… And when you reach forwards in the battleline for a dramatic murderous blow on that barbarian in front of you, the last thing you want is his buddy to bring down a CLEAVE call on your unprotected forearm as it comes in his easy reach.
Conversely, CLEAVE and IMPALE only do 1 point of damage if they hit head or neck (this is to make it mechanically unprofitable to perform head hits, as well as discouraging huge two-handed blows to the noggin…
…I have fallen over when recieving an IMPALE to the head. Once while playing a barbarian orc, in the woods… I snuck up on their rearguard, charged, and an archer turned and fired, 30ft away, eyeshot! Perfectly safe, but I felt it was awesome enough for an insta-kill. And again on the battlefield, playing my character, wearing a medium helm. This giant lobster monster bigger than I was grabbed me round the head with a claw and was thrashing around while it died under the blades of my friends. It collapsed, I collapsed, I asked the monster “what damage were you doing with those claws?” “IMPALE by blow”. And I decided to go on my death count then. Because come on, that so should have been a horrible fatal head-crushing injury! Character survived, I got jumped on by physics, walked off the field to buy a new helmet, and had lobster for dinner.
… yeah, LARP is a mechanism for generating stories 
Anyway, re expense… if you’re playing an orc, you can get away with not pristine and mirror polished armour slightly more than in, say, Highguard or Dawn. So have a look around the internet for any kit that’s being sold off. Look through those traders. Ask friends if they can lend you something for that first event or so, to give you time to have a look at everything.
However, armour will last. It will likely outlast you character and the Empire system. Unless it’s something really specific (chainmail with your characters heraldry built into it), you’ll be able to use it for the next character, and the next, and the next. And if you end up selling it on, it’s got good resale. It may be the most expensive single purchase you make for your character, but it’s an investment.
Look, for example, at the knight in the illustration here:
https://www.profounddecisions.co.uk/empire-wiki/Gryphonsoul_Aegis
Simple, generic, re-usable, looks good (Alas, it didn’t save her, but that’s another story…)
You’ve got over 5 months to sort kit out, plenty of time to go for a cheaper option if you can find one. Just don’t get over-ambitious and decide to make your own from scratch…
with regards to armour, would shoulders and perhaps greaves lock it in as heavy armour? Just trying to figure out a budget here as i’ve already seen it’s quite expensive.