The terms are the same as for the US: you have 30 days to begin watching a film after you rent it. Once you begin to watch it, you have 48 hours to complete watching it. During that 48 hours you can watch it as many times as you wish.
You do not have an option to buy a rented film at a reduced cost. If you want the film, and it's availble for purchase, you will have to pay full price. Some films available for rental cannot be purchased at all, and vice-versa. It all depends on what the movie studio was willing or able to license.