The developers use Softimage XSI to create the models for sins. There is a freeware version called XSI Mod Tool. However models saved from Mod Tool may, or may not convert to mesh format with the forge tools. I have had no success converting from mod tool, but others in the sins community have had success.
There really is no "free" way to mod sins. However you can do it "cheaply".
Modeling for sins is easy, and you can use virtually any modeling program available to create your model. Milkshape 3d is dirt cheap, and wings 3d is free IIRC. I personally use 3d studio max. However for best results you will need to finish the model in XSI according to the modding documentation provided with the forge tool set. So if you plan on modding strictly sins then XSI mod tool would be the best place to start. We cant walk you through learning how to model. That is something you will have to research yourself. There are plenty of online tutorials for XSI.
Texture creation/editing for best results use Adobe Photoshop. However i hear Gimp is a good freeware image editor.
The game is very moddable, but there are some hardcoded aspects that may, or may not annoy you. There are only 9 capital ship slots in game. Phase lanes are hardcoded. Gravity wells are hardcoded (only 1 planet per gravity well). Weapon firing arcs are hardcoded, and non-adjustable. Entrenchment lifted some restrictions on ship movement. As far as replacing the ship models, and textures themselves that is very easy to do.
Firing points are very abstract. the game only sees forward, left, right, and back firing arcs. IIRC all points fire from a 180 degree arc, and can only be placed pointing 0, 90, -90, and 180 in XSI. We also found out the hard way it sees "Up", and "Down" as well. If you leave the hardpoint alone it will fire at ships on top of you. If you flip a hardpoint upside down it will fire at ships below you.
There is much you can do in sins, but as far as "full" freedom? I never seen any game that had total creative freedom.
