Lean Startup is great but you should realise the value of investing in good design. Don't go down a rabbit hole of 'big vision' design perfection straight away, sure (a $50k set of Photoshop comps before you've got product-market fit isn't a great idea) but you're not going to get very far with a fucking Unbounce template.
Hire a good designer—one who knows how to do 'early stage startup'—and flourish.