There's an interesting world of combining robots out there. generally, a combining mecha figure will make some sacrifices, generally in articulation. I'll go through a few choices in increasing price ranges.
First off, a good, "cheap" combiner would be the Transformers Combiner Wars/Unite Warriors stuff. The best thing about them, if you're not attached to the existing characters, is that you can buy a set of four "limbs" and one torso to make a combiner, so you can buy it in small increments. There's also somewhat cheap boxsets available. Proportions are a bit wonky, but the components having individual bot modes is
nice.As a starter, I recommend G2 superion.
