Once you set up your products in E-junkie Seller Admin, we provide you with ready-made HTML codes for your cart buttons, so you would simply copy the codes from us and paste them into the HTML source of your own Web site pages, wherever you want our buttons to appear among your own layout, text and images.
You could set up each model as a separate product, so each model gets its own Add to Cart button, or you could define the models as Variants of a single product, so the buyer would select their preferred model from a menu before they click Add to Cart. See more info here:
http://www.e-junkie.com/ej/help.variants.htm