Well, my first suggestion is to get the site redesigned. It looks like it was made about a decade ago. Putting that aside - Simple Checkout will be fine if people will only ever be ordering one item. If they can order multiple items, you're going to need a shopping cart and DPM at minimum, so the site can dynamically set the amount they're paying, and if you're going that route I personally prefer just getting an SSL certificate and running the charging process entirely from the site with AIM. But either method is fairly easy to implement - the key is having a shopping cart that either supports Authorize.net or can be easily modified to use it.
As it so happens, I'm designing a shopping cart for a client that uses Authorize.net, and if you can hang on 2-3 weeks, I can modify it for your needs and implement it for a reasonable fee. Assuming you can afford a site redesign, I can probably help you with that as well. Just go here and fill out the contact form, and include any info / ideas you've had about improvements that could be made to the site.
http://www.fentonwebdesignfirm.com/contact-us.php
If you're interested, I have some past clients listed here. Can also supply more examples via email.
http://www.fentonwebdesignfirm.com/portfolio.php
