Sarah Heart-Rose of the Black Heart Roses mercenary unit is described in FM: Supplemental II as a collector of BattleMechs. It says she renovated a Javelin pulled from a scrap heap and renovated it at age 15, then later an Assassin at age 18, then later acquired her "current' mech a Black Knight. When she inherited her father's mercenary command it says her collection grew considerably.
Her unit, a full battalion of functional combat capable 'mechs, has another battalion's worth of 'mechs in various states of repair. It also says her unit has Adam Steiner's Axman, and Grey Noton's "Legend Killer" Rifleman. A Victor sold for scrap by Katherine Steiner-Davion, an antique Striker and a couple rare Spartans.
The unit is better known for collecting 'mechs than anything. Painting them up in the colors of various famous units. The unit even sold their only dropship, a Union, to supply the funds to buy more 'mechs. The unit has built up enough technical support to maintain a regiment's worth. At one point they tried to acquire Morgan Kell's Archer from the Kell Hounds as part of negotiations for a contract, but the Hounds refused.
This definitely toes the line between an individual person and a mercenary unit. But it does sound like it started as a person, before she inherited her father's command she had two 'mechs of her own that she renovated or acquired on her own by the time she was 18.