Brand doesn't matter but fit does. Make sure the coat fits and if it doesn't fit well get it tailored (if its too big) seriously will cost less then 10 bucks, but a well fitted coat will make you look super expensive and impressive :).
Navy is always in style. ALWAYS. I'd have to see a picture of the red/green plaid, doesn't sound show ring acceptable though.
The grey should also look good.
Gordon Wright once told a student to take up swimming because he would never be a good rider, that rider was George Morris.