Regarding your shares, the lender most probably won't use them as collateral. You have the option of selling them to raise further funds (and therefore decreasing the mortgage amount you require) or alternatively if you don't want to sell them in the current market, you can advise your lender/broker that you have them, their approximate value etc and they may look more favourably on your application. This is especially useful if you're borderline.
You're first port of call is the lender/broker. Once you know how much you can afford to spend, you can then start looking at properties within your range.
When you are assessing the monthly repayment affordability, don't forget to factor in additional expenses such as life assurance and house insurance, management fees etc.