ARMORED CORE VI FIRES OF RUBICON

You do not need to own all parts at once. You can buy all weapons, get the separate achievement for that, and sell them all back. Then buy all frame parts, get that achievement, and sell them all back. Same for inner parts. You should have enough to buy out a single category after your 3rd playthrough and way more than enough if you're going for all S ranks.

Note that you will still need to collect all combat logs, find all of the hidden containers, and beat all arena opponents. Doing so will require you to play all mission variants and get all 3 endings. There are plenty of guides elsewhere for those, so I won't take up any more space here.