You need to provide the designer the limits of what Marketo pages can do. Make sure the person coding it has the html from a Blank Marketo LP Template.
Make sure they know the DIV tags needed for editable sections.
I agree with Edward that you need to constrain the designer to make a page that isn't too fancy.
Personally, I go with any small agency that has experience with landing pages and can code+design a page to your needs.
Marketo can do this for a fee. Most small graphic/web shops can help you. Or you can have a firm like LeadLizard, Pedowitz, LeadMD, etc. handle this for you.