Rowdy is a power armor specialist merchant. She sells the Atom Cats custom paint job schematic and stocks fusion cores, a power armor frame, and a full set of Atom Cats painted T-60. She can also have parts of all sorts of power armor types, ranging from raider to X-01, but availability is limited by the Sole Survivor's level.
Rowdy will repetitively comment "we could use a few more suits around here" when around the player outside of direct dialogue. Purchasing or retrieving any number of power armor frames and keeping them in/around the garage doesn't seem to satiate this desire, which she will continue to express no matter what may change.
Power armor mods sold by Rowdy are of the X+ variety. Placing any of these mods in a workbench inventory will allow unlimited application of these mods to any corresponding power armor within any connected settlements.