In terms of builds; it determines whether you want to be a heavy sniper with a shit ton of damage
Or whether you want to be a speedy little shit who runs around shooting people with decent damage with a lot faster reload speeds.
Or you can be similar to both and sit in the middle
There was a big reason that a lot of Xbows in Vanilla chose the Heavy Xbow over the Siege, because of the extremely fast reload speed in comparison, and it's the same with the Arba and Heavy Xbow. However, Arbalest now has a big advantage in that it is a 1 shot headshot with both the xbow and the bolts being +3
It's all dependent on accuracy to be honest with you, if you think you're accurate, go with a strong build that you can use to deal out tons of damage, but if you're not that accurate go with a lighter build.
As for Xbows, I'd say the Light xbow, Heavy Xbow and the Arbalest will be your main choices to go for, as they're sort of for each category.
I don't really play a lot of Xbow so I couldn't give you an ACTUAL build per se, but I can offer what I know from general experience of playing the game.
As for armour, you kinda want to go for something light, but still decent protection, the Lamellar protection is good all round for Xbows as its Light but good at the same time, however, I prefer the Heavy Aketon for more mobility.
However, body armour is less important as an xbow, having good head protection is a MUST (This is due to the fact that when you reload, you massively expose your head) I recommend the good ol' Kettle Helmet, it's always been my choice when I played Xbow, and I think from the last time I saw it, the Aketon pairs well with it.
Mail gloves are decent
And boots don't really matter, I usually pick the Norman Mail Chausses because they look good.
