I can only comment on the scar and stretch mark issue. I have been using a dermaroller for both, and getting really good results. It can be a little painful, but the pain is quick and not lasting.
I was unlucky to get stretchmarks worse than others, but strangly lucky to respond to the therapy very well, some are almost gone and my C-section scar is disapearing nicely. It also tightened up my skin.
Not everyone will respond as well as I have though, but some get really great results. It's worth trying.
Lot's of info on the web, there are a couple of forums out there to get support from. I buy the cheap rollers from ebay though, I don't buy from the forums or the brand names.