For information about buying a computer, you can check out
MIT's Computer Buying Advice. I think some of the suggested systems are out of date, so you might want to wait until later in the summer when they'll update the page for incoming students. Or you could call the Computing Help Desk and talk to Presales Consulting. You can also find links to where to buy computers with MIT's discount.
As for whether you need a computer right away or not, its quite possible to survive without a computer. There are computers all over campus that you can log onto using your kerberos username and password (what you set up for your email). I think most dorms have some Athena computers as well.
The
Wireless Network Coverage map shows that most of the campus has wireless access, including all of the dorms. When you get on campus and your computer connects to the MIT network, you'll just have to register your computer and you'll be good to go.