Pride really needs a 'mech that looks superficially invincible but has a major flaw (remember, "pride goeth before a fall..." also, the tale of Achilles, invincible everywhere but his heel).
Or, since it has the Pride Aesop built in, you could give Pride an Icarus/Icarus II...
Wrath ought to be melee-focused, maybe a Kodiak or an Axeman? Or better a 'mech modified with strong mêlée weapons and TSM, but no other weapons.
I've got no ideas for Lust (given that the usual definitions don't lend themselves well to BattleMechs, and I'll avoid the low-hanging fruit of just mentioning Canopus and calling it good...)
Sloth? That PHX(-99?) from the Caveat Emptor joke TRO
Avarice? The rarest and most expensive 'Mech available. Maybe an Exterminator with the cloak systems still intact.
No ideas for Envy either.
Gluttony? Not sure, maybe a vehicle that canonically is known as a gas-guzzler?
Despair? A Manei Domini Omnimech. Those 'mechs spark despair often in their opponents. When they're not sparking outrage.
Really, any Mech associated with the Seven sins should be a double-edged sword, because in theology, remember that the seven deadly sins are not great things to indulge in the long run.