Re charity shops: look for braid, look for costume jewellery, look for simple, bright patterns. Check the womens sections as well, for materials.
In terms of the heraldry, you can do pretty well with fabric or acrylic paint, just painted over (repeatedly, sometimes) onto a piece. A sash or a belt tag if not a tabard.
So your character plan is (a) join Dawn, (b) attain a rank of Noble in House B (c) marry the pretty girl from House A, from a position of equal rank… sounds solid. Far more careful Marcher than flamboyant Dawnish, and likely to work.
It probably won’t be your last event, but being frugal for the first go is very sensible.
I was asking re the armour because of the rules on these things: full torso coverage + another location = a full set, and for heavy that triples your starting hit-points. Without a bit more torso coverage, you might not quite make the standard, which would be a shame as you’re so close (although the armour pieces physically protect you against some of the nastier damage calls). There are various possibilities… still, not an issue.
I would advise asking about, on here or the Facebook page for Dawn, and see if there’s a House you might like to tag on to for the event. Maybe to ask them for a Test of Mettle, maybe just to hang around with them: somewhere with a tent where you can put your things down, sit down for a drink, use a point of reference and the like, is very useful. It also gives you a group of people to socialise with, to whatever degree you like. Plenty of time to think on that.