The H-brace is a part that goes under the body... which I have no personal experience with.
As for the Strut braces... some fit, and some don't. Depending on the design of the strut brace of course. I found a cheap local strut brace, similar to the Cusco STi unit, and it didn't fit because of firewall clearance issues.
As for the rears, any WRX rear bar will fit... technically. The sedan bars require trimming of the mount plates, but the WRX wagon units fit fine and are typically of the quick-release variety.
Did you end up getting the bars yet? I had ordered a the front and rear a while ago, I had to do alot of drilling to expand the rear bar to get it to fit, i ended up getting rid of it as its structure was no longer solid. I have heard the the older 93-01 impreza bars fit in the rear? not sure though.