Infantry got ignored in the beginning because what's the poster child for the game? The CAV. I want robots and I want all the robots. Big, small, tall, short, thin, fat, nimble, sluggish, heavily armed, lightly packing, able to make a pinpoint strike or just going to waddle over and sit on the enemy's house, we don't care we want the robots. Then you find out CAV's cost a bit TV pointwise, and you realize tanks are good too. And aircraft. And infantry can sue up that last 400 points you can't buy a squad with and then you're 4 orders into Talon Games and the credit card company would like a word with you.
#1. Data card pdf's can be found here CAV: STRIKE OPERATIONS Downloads (talon-games.com) All the infantry data cards are under OPEN as all Factions share the same stats/models for Infantry. Also listed are Datacard pdfs for all Factions and documents listing all a faction's models type and points cost. May I also recommend a copy of the Cheat Sheets. The Special Ability one is almost Mandatory.
#2. As far as kit-bashing or proxies go it's open season. Some make their own CAV's out of existing ones, some use models and other parts from other games. Some just buy things similar to what they want and say, "This is model x. Learn it, Love it, Live it." CAV is 10mm or N-Scale so as long as it is roughly the right size and can be easily identified it shouldn't be a problem. If you have access to 3-d printing Thingiverse can be a source of a mess of parts and vehicles.
The last game I played my Recon squad of Nomad's were played by the Templar 99th Regiments' Mamba recon squad. We thank them for their service.