The achievement require the collection of all the Battlemech's chassis released at the original game sale launch, so, of all the nine different SLDF Battlemech chassis currently available in game, only the Highlander HGN-732B and the Atlas II AS7-D-HT are needed in order to complete it.
I'm also quite surprised at the base purchase price stated by
@Chaosticket for an Atlas II AS7-D-HT chassis at the black market as IIRC I paid my second one 21m C-bills or so, not 190m - and that happened one of the several times when my original outfit's reputation with Pirates went back from a terrible "Loathed" (i.e. +1000% markup) to a more manageable and affordable "Indifferent" (i.e. +10% markup).
So, unless I got it completely wrong, or the last patch applied some worsening price conditions to the black market of which I'm not currently aware of, I'd assume the 190m C-bills price is indeed for a "Loathed" status with the Pirates themselves as, at least in my first and only catch-'em-all, do-it-all pokeMech campaign I've managed to collect all available SLDF chassis twice, of whom only an HGN-732B, an AS7-D-HT and both GRF-2N were collected "free of charge" due to campaign and flashpoints rewards, while the rest were bought either as complete chassis or as multiple salvage parts and then assembled in their corresponding Mech. But I never spent more than 30m c-bills for any of them.
Finally, I agree too that Lostech and SLDF Battlemechs don't need to be actually deployed by the OpFor in order to be legit to collect, as purchasing them in any Black Market shop is as much legit as salvaging or otherwise acquiring them on the battlefield.