Elaborate on 'putting stuff together'
For Blockland I believe Sketchup would go really well. You would need to get the pro version in order to export to Milkshape, because you'll need it to add the joints and flatshade.
A modelling program also depends on your style. If you like to have a simple interface with decent functionality, I'd go for Anim8or, Sketchup, Wings3D, Milkshape or anything of the like. If you prefer a somewhat more complicated interface but a butt load more functionality, go for 3dsmax, blender, etc.