Probably if you lack some advanced modelling skills, the best idea is to start modifying an existing model. For example, you can use Misfit3D (freeware program) to open and modify Lightwave objects, which is a format that Silent Editor can output SH3/4 models from the game.
Give it a try, just altering proportions and size of existing units can make them look very differently, and you can also combine superstructures form different units with Silent Editor, playing with the nodes
