Iron Wind Metals are a great source of 6mm mecha. Their "Battle armour" for their 10mm Battletech line are a perfect fit. Just look in their Online Exclusives section. The range is huge and has more options than pretty much all the 6mm manufacturers combined.
I found the prices (50c-$1ea) good, but postage to Australia was steep - $17+ $9 packing fee meant that the postage added 50% to the cost of the mechs. As you can see the 10mm "battle armour" make perfect 6mm mecha.
More Hound Dogs as part of the OpFor.
I also really like the Brigade power armour. It has a real MaK vibe.
The infantry is also quite good, and is well-priced.
A decent-sized 1:300 force will cost around $30 - 20-30 vehicles for about the cost of 2-3 10mm Heavy Gear models or Battletech mechs.
The buildings are also Brigade models. They are also quite well-priced.
Even a small 4x4ft table leaves plenty of room to maneuver in 1:300 scale....
I'm also working on a way to abstract the zillions of hitboxes in Battletech using the bell-curve percentages of 2d6 (the possibilities are quite interesting - I might do a separate post on this). I've tried the new Battletech fast-play rules "Alpha Strike" and found it too simplistic. I still want to know if individual arms and legs are blown off - but I don't want to have to "scrub off" say all 40 hitboxes of leg armour before I can actually damage the mech's legs. It's kinda silly really - in reality shots either penetrate armour or they don't - they don't have to remove every last bit of armour before they can start to do internal damage. I'm also experimenting with using the Fistful of TOWs modern rules system (it has vehicle design rules that I can stretch to cover mecha).
Stay tuned for more mech action...