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Very basically, the designs have to look great (modern look, clean code, responsive design is a bonus) and have a certain level of SEO-friendliness on launch. I think it's very important to know which CMS they use, whether it be a 3rd party product or a custom solution, to determine how SEO capable and customizable it is.

Thanks CleverPhD. Exactly what I was thinking, just wanted to have someone else confirm. Appreciate it!

Daniel, There is an easy way to Lazy Load images using JQuery. However this means changing the image's src attribute to nothing and replacing it later on. Do I understand correctly from your answer that it is not recommended? (I assume because Google misses these images). Please advise (and also provide an alternative if there is one for a page with more than 100 images - even though each image is approx 8kb). Thanks