Hi Damien,
I have not sold any yearlings in the standardbreds but have a bit of experience with thoroughbreds. In my experience yearling preps can range from 6-12 weeks depending on how much you are willing to put in. Most preparers I know charge a daily rate plus sales costs. Commission from sales in unlikely if you are paying a daily rate with sales costs. However like with anything everything is negotiable and there are different ways to fund/pay for things.
I recommend you give a couple of the good farms a call and just ask them. Just have a look through the APG catalogue and you'll soon work out who the main farms are.