I've done it twice. Normally for first time buyers only, but I was able to do it a second time due to the breakup of my marriage and the fact that DH2 was a first time buyer.
It really depends what area you are in as to how often houses come up. My first time was in SE London about 4-5 years ago, and there were loads of properties (some new build, some resales). Second time was in Cheshire, and there was less available.
While I think rents will differ slightly depending on which housing association your shared ownership arrangement is through, I believe it's normally something like 2-3% per year of the share you haven't purchased.
Will use my situation as an example.... if you purchase a 50% share of a house that is worth £150,000, you get a mortgage for £75,000. The part you don't own is also worth £75,000. 3% of that is £2,250 a year, or £187.50/month. 2% would work out to £125/month. In my current situation, we pay £132 in rent, so just a little over 2%.
Can't answer your question exactly without knowing what the 70k mortgage represents as far as a percentage share of the total worth, but you should be able to get an estimate using that formula above.