Think of armour as a series of levels, based upon how many average hits in melee it takes to kill you:
Body armour (gloves included):
1-hit armour - Up to about 25 Body Armour
- Most things will one-hit-kill you
- Ranged will chew you up
- Bumps hurt a lot
- No WPF penalty
2-hit armour - Around 30-40 Body
- Less likely to be hit than the 1-hit armour, because you are not dressed like a peasant
- Ranged still hurts
- Bumps don't hurt too bad
- Minimal WPF penalty (body armour weight will be about the highest you want for archery)
3-hit armour - Around 40-55 Body
- Melee will start to glance on you if you can avoid their speed bonus
- Ranged don't want to shoot you
- Bumps are mostly ignorable
- WPF loss is increasing, inadvisable level of armour for ranged
4-hit armour - 55-65 Body
- Lots of glances
- Non-heirloomed archers, sub-5 Power Throw throwers, and Light Crossbows will be ignorable damage
- WPF loss is significant, you will swing slower than the other guy
5 or more-hit armour - 65+ Body
- Everything glances if you can use your momentum correctly
- Ranged ignorable unless you get shot in the face while running at them (assuming you have 55+ helmet on)
- Massive WPF loss, equivalent of a few Weapon Master skillpoints
Helmets:
Light (up to 25 Head Armour)
- Less likely to be insta-killed by melee swings
- Ranged is still a headshot kill in most cases
- Minimal WPF loss, probably the most you'll want as ranged
Medium (25-45)
- Will save your ass from ranged time and time again
- Can take a few face hits in melee
- Pretty big WPF hit
Heavy (50+)
- Most ranged survivable or even ignorable except the high-end throwers and Arbalests
- Glance point for melee
- Large WPF loss
Boots - Wear about the same as your body armour for infantry. Less as ranged, more as cavalry, salt to taste.
As for heirlooming your stuff:
Gloves - Your best option to self-loom. Always tradeable (except the Heavy, max-armour ones). Mail Mittens are a solid choice, usable by all classes (and peasants).
Body - Only self-loom shit you want to keep or know you can trade away down the line (ie no plate, samurai shit, peasant armour). Best to try and buy a +1 armour within the range you want. Byrnie is a great option, great light-medium armour that is usable by all classes (not great for cav though).
Helmet - Self-looming is pretty much your only option. Makes a HUGE difference with a heavy helmet, negligible results for light and medium. Don't loom until you've got body, weapons, gloves, horse (if applicable).
Boots - Same as helmet, except leaning more towards cavalry players, who will notice the most difference in survivability (and again, only at the upper echelons of footwear armour values).
Other random tips:
- Plate armour is underrated if you can learn to counter speed bonus momentum.
- Wear less armour if you have a shield.
- Only trade armour for armour (body for body, gloves for gloves, etc). It can be tricky to re-aquire armour slot heirlooms, and they make a big difference.
- Wearing more armour could mean you earn more multipliers, so heavy armour isn't just for "special occasions." Plate is financially viable as long as you aren't using horses, poleaxes, or greatswords.
- In case you missed it above, range are far less inclined to shoot the guy in heavier armour. Wear more than the guy next to you, and they shoot him.
Upon reflection, I should have just wrote a guide section post.
