Battletech "unseen" Battlemechs are based directly on Robotech so any of the Bandai/Imai destroid and veritech kits in 1:100/1:200 are perfect. Fighting Piranha Graphics do decals for almost all the Inner Sphere houses and merc regiments, plus all the Clans.
The other alternative for big scale models are by Armourcast, but at AU$250 a piece, they are expensive.
Horizon also did a 1:35 Timber Wolf (Madcat) but as that goes for around $400 on eBay....
Here is my 1st Federated Suns Armoured Cavalry MAD-3R Marauder c3028 (Bandai/Testors 1:200 Robotech Officers Pod).