So it's cool that you are thinking ahead to using this armor in the SCA. That being said, it might be a good idea to check back here before you buy something to make sure it will meet the requirements of rattan combat.
The helm I think you are talking about is called a "great helm" Here is a pic I googled:[img]http://i371.photobucket.com/albums/oo159/bhaiduk/blackprince.jpg[/img]
Depending upon who told you you want a balck prince helm, you might be talking about a bascinet, as the black prince's effigy has him wearing a bascinet, but the great helm was hung above the effigy. The great helm was actually worn over the bascinet. However, I'm like 99% sure the "black prince helm" you are talking about is a great helm. (If I'm wrong someone will post here otherwise)
Bracers, or vambraces are pretty standard. There are two pieces that strap together around your forearm. One other version you might consider if you are interested in transitional period armor is a splinted vambrace. That is leather with steel splints riveted to either the outside or inside.
Elbows come in all kinds from simple one piece to multiple lamed, and ones with and without wings. And they were just about all used in the transitional period.
I don't realy know much about breast plates, but they did start to come into vogue in the transitional period.
So, your history isn't so bad. Plate armor generally wasn't used in the crusades era. With the exception of helmets, most everything else was mail, cloth, leather, or smaller pieces of steel laced together called lamellar.
Mild steel is good choice if you want it to look weathered. The simplist way is to leave it out in the weather

pun intended.
Anyways, back to what you really wanted. Normally I would send you to Mad Matt's armory
http://www.madmattsarmory.com/, but he is not taking orders at the moment.
Here is a short list of armorers that I could think of off the top of my head. Most of them have prices listed on their websites. My apologies to anyone I forgot.
Stone Keep Armory
http://spiers-saddlery.stores.yahoo.net ... andar.html
Bokalo's Armory
http://bokalosarmoury.com/
Illusion Armory
http://www.illusionarmoring.com/ (they have a great helmin your price range)
Rough From the Hammer
http://roughfromthehammer.com/
I've only ordered from one of these guys, but if you post here asking for a review before you buy you can get a good idea of whether your getting screwed or not.
I hope this helps. If not, ask more questions!!