Haha, I saw C+C, and I initially thought you were trying to build a close combat oriented chapter! I began reading your initial post, and thought "Where's the close combat stuff?" Then I finished reading it and got it.
Anyhow, here's my thoughts: First, fighting "defensive battles" and raiding to recover lost equipment are kind of antithetical priorities. What you might want to say instead is that they try and avoid getting involved in protracted battles, preferring to hit and run, leaving ambushes behind them to slow enemy pursuit. That would let you play two kinds of lists: a "raid" list and an "ambush" list. The ambush list could be considered kind of defensive, because you're letting your opponent come to you and then hitting him when he's not expecting it. Doesn't lend itself very well to the first couple kinds of missions, but would lend itself well to Kill Point missions. Or, you could home-brew some missions, where the opponent gets points for every unit in your deployment zone or units of yours destroyed, and you get points for every unit kept out or destroyed. Anyhow, Scout Bikes would be good in your ambush list, for their booby-trapping ability and all.
Anyhow, I'm thinking that one aspect of your force could be that every infantry unit that isn't either a jump pack unit or bike mounted has to be mechanized in some way (unless they drop pod or teleport). So, Rhinos for all your Tacs, and even for your Devs (maybe a Razorback for your Devs, if the squad is small). I wouldn't necessarily use Chaos vehicles for this (definately wouldn't, actually), but you could use some plasticard to represent crude repairs done in the field as opposed to the chapter armory. Also, Chaos isn't the only force they could scavenge Rhinos from. They could have scavenged Rhinos from the Inquisition, Sisters of Battle, or the Adeptus Arbites. So, you could throw in some Arbites Rhinos with hasty repair jobs and plates welded on the side with your chapter heraldry. Probably Arbite Rhinos would be most available to be scrounged up. You could probably afford yourself one Predator, but make it a dakka Pred with a lot of patchwork repairs - I figure dakka Preds are easier to maintain than lascannon Preds or Vindicators.
I'm very interested to hear about your progress on this. It sounds very characterful, and it could be striking visually.