The needling initiates an inflammatory process for the first 3 days because tissue has been wounded. After this, new collagen is laid down in a disorganised haphazard way. From 4 weeks up to 2 years after needling, this new collagen will gradually mature and become the organised, strong collagen that we want. We get scars where there is excess accummulation of collagen that has formed irregularly. Post needling, during the process of the collagen maturing, the irregular layers that form the scar will be broken down. Needling is carried out at monthly intervals to give the skin time to go through these phases.
Environ do training, Boston Medical also train and their rollers are very well priced. I've done needling on 20 year old stretch marks using 1mm needle and anti-ageing using 0.5mm, you don't need to go for the deep needles.
I also recommend the best book on the subject: The Concise Guide Dermal Needling Medical Edition by Dr Lance Setterfield.