[AkibaSilver]

To build a complete figure, you need 1 Cross Armor and 1 Action Body. The exact one you want depends on the figure. For example, there's a Generic Woz Action Body, and you can buy Cross Armors for his different base forms. Ginga Finally is more different, so it has a specialised Action Body as well as Cross Armor.

For Decade Armor in particular, its base Action Body and Cross Armor (for the standard Zi-O mode) were released in Wave 5, so January-ish time, while the RabbitTankSparkling, Might Brothers XXL/R had dedicated Cross Armor and Action Bodies come out recently. Since the Action Bodies have the legs, and the main change for Decade Armor is the legs, they all have different ones. TaJaDol and Grateful are coming out soon-ish, I've been too busy to keep track of So-Do lately.

So yeah, my best advice is to pick forms you like, and then maybe come ask here as to what you need to build them. If it's an upgrade form, you can probably just buy a specialised cross armor and an action body and be done with it.