check the Esis calculator you should get with the offer.
it will show you the cost of credit and the repayments over the term of the mortgage.
it will also show you the total amount of interest you will pay back on the mortgage, if the rate was the same for the term of the mortgage.
oh and to answer your question, you can get as many letters of offer as you want, just remember there is a time on the offers limit that will make them null and void when you reach that time.
normally 3 months from date of offer at the moment.