Thinking of buying the house from my parents

Hi,

I have a question for you folks. I am currently living with my parents at their condominium home. They are thinking of selling their house (other house in the area are selling for around $200k) and I am thinking of buying from them. The advantage of that is they can continue to live with me and I do like the house and area. It will be my primary resident and if I purchase the house from them. My questions are follow:
1. Is there's a restriction as to how much they have to sell the house to me for?
2. Since it would be my first home and primary resident, could I use my rrsp and qualify for first home buyer tax credit.
3. What's the tax implications?
4. Is there any downside to my parents for selling the house below market value? (they are both retired)

Thanks in advance for any advice or answers to the above questions.