To be clear: there’s no such thing as the “best” or “right” armor. With that:
How much room should I make when ordering a jersey?
Specific to these CanvasMX jerseys, I cover this in the jersey post. Reread it under IMPORTANT FIT NOTE.
What armor to buy?
This is so personal, and depends on your risk tolerance, how much coverage you want, and what your heat tolerance is.
A better question to ask is “What’s my minimum level of coverage that I’m comfortable with?” For me (Gino), it’s CE Level 2 for chest and back, and CE Level 1 for elbows and shoulders for off-road riding. You can read more about CE Level definitions here.
From there, it comes down to A) what is your budget and B) can you try it on before buying it? Fit is everything for long days on the bike.
What’s reasonable prices?
This totally depends on your budget. There’s simply not a right answer. That said, armor is quite often on sale at places like Revzilla, RMATV, etc.
Brands to avoid? Preferred brands? Price range? Used a good idea?
The brands I use are 100% based on how they fit me, how they perform in hot weather (I do terribly in warm weather), and level of coverage. For me (a sample size of one), spending good money on protective gear is more important than the bike itself; If I had $4,000 to spend, I’d spend half on a bike, and the other half on protective gear (boots, helmet, armor, etc.).
That said, I’ve had a good experience with many Leatt products, and to an extent, Alpinestars products. I’m currently testing Mosko’s new armor, and so far am unimpressed (even though I love other stuff they make.)