1. Can an international student over 21 yrs of age buy any kind of property in Canada ?
Yes, if he can afford it
2. Is there any restriction in buying the commercial property under student visa for an international student ?
Some provinces may have restrictions, see http://www.assignmentscanada.ca/buyingincanada.html for example
3. Is there any restriction, different city or state wise, for buying or selling the property ?
Also according to the link above, there can be some taxes when you sell the property if you are not a resident in Canada
4. If a non-Canadian can buy the residential or commercial property then, under the following cases what implication he has to face ?
a. Can he apply for a loan in Canada for buying the property or he has to take loan from his home country ?
He can apply but with no credit in Canada and no steady job, it may be hard for him to find somebody who will loan him the money. However, the more money he has for down payment, the easier to get a loan for the rest, of course secured by the property.
b. If he gets a loan from Canadian bank and after a yr or so he leaves the country due to any circumstances then what can be the problems which he has to face regarding the rights of the property or how the bank gonna treat this situation, etc. ?
If you get a mortgage to buy a property and you stop making payments whether you are in Canada or not, the bank will take your property from you and auction it to get their money back. If you leave Canada but keep paying your mortgage, the bank couldn't care less. If you have to leave and can't pay your mortgage any more, you should sell it before you go.
c. Will he get the visa in future after leaving Canada to his home country on the basis of his property over there ?
Having property does not give you any status in Canada and will not make it any easier for you to get a visa in the future.