The ability to purchase armor is a good idea. I agree that maintenance should not play a part of it. Instead, you could have the armor degrade every time it has prevented a wound or reduced the impact of a wound. A noble would then need to choose whether to fight in a little scratched armor or buy a new one (and pay full price). The armor would have several stages of damage, all of which would reduce its effectiveness (and shininess) a little, but never below a lower tier armor. It would be assumed that the armors would be repaired between battles, but that they would never return to their original splendor.
There would be two ways to look at it. Some cultures would prefer to wear armors with marks of many battles - proudly like scars. Some would seek maximum security or shiny impressive looks and spend gold towards that end.
To sum it up, people would not be able to just purchase the top tier armor and keep it in splendid condition indefinitely (unless they avoid battle). The truly rich could maintain best protection and style, but with a rather high cost attached. Optionally people could just purchase whatever armor they can afford and forget it for quite a long time. How often you do get wounded anyway? If it took around ten wounds or prevented wounds to really hurt the armor, for most people I imagine it would be quite awhile until they felt a need to replace it.