The books from the guy that created Clickfunnels are pretty good. I think Dotcom secrets is the one that specifically talks about landing pages but it could be another one.
I'd also recommend Clickminded. I did their SEO course which is great. They have one on Sales Funnels , Content Marketing, paid ads. Haven't done them but always been impressed with anything else of theirs I've done. Sometimes they run big discounts for the courses on AppSumo so check there.
I wouldn't approach a designer until you have the copy sorted out. You have the raw elements - good number of testimonials, sales pitch. Just needs some shaping - like opening header, sub header, bullet points. Get it all formated on a Google doc so you can give it to the designer and they can design but they have the copy to go off of.
You can also look at the websites that just do landing pages, like Unbounce and Leadpages. They handle the hosting, they're super fast, they have lots of templates, relatively easy web builder, easy to A/B test or create different landing pages for different types of audiences you're going after.