If you are a big company that expects to make a high volume of sales via your e-commerce site. You'll need to buy good quality webservers, preferably running linux with a load balancer, or just have a look at cloud webhosting as this will allow you company to only pay for the resources that you use.
I'd always recommend using Linux with Apache with nginx for your platform.
PHP is a programming language normally used to generate the content on websites, this fits in well with MySQL which is a type of database. The PHP would normally get a list of products from your database and display them on the webpage. This database would also store orders and user details, plus any other relevant information your business might need.
AJAX is there to make your site that bit more user friendly. For example, if you use facebook you'll notice it has facebook chat and also content that appears without you having to reload the page. There are a few more functions to ajax aswell such as, submitting forms, etc...
When it comes to the development consider a few of these:
How long it takes the user to get to know the site and its layout.
Is it simple to navigate
Does it fit your needs and the user's needs
The timescale of the development
How easy will it be to integrate it into your current systems (if any).
As well as selling just products on there, is there another source of income from it (e.g. Advertising)
Your target market
How much it appeals to impulse buyers
Is the checkout procedure going to be simple
Is it search engine friendly
A perfect e-commerce site is a site that looks nice and professional, completely intuitive, and allows its users to find what they want in a matter of seconds.
Look at Amazons layout, CPC's layout, Ebay's layout. They all have something in common.
The minute the page loads you're shown a search bar, list of categories, a list of offers and new products. You'll find this with a lot of e-commerce sites.
Hope this helps,
Adam :)