You really want to do it yourself. If something happens, you want to know how to fix it yourself. As well, save your 150 bucks and get some solid safety things, like a custom sprocket adapter, dual brake pull lever, new lights, etc etc.
I don't charge to install a motor - but I do make whoever is there pay attention the entire time so they know how to maintain things and so they know what's what on it.
Just hang out in these threads, ask a lot of questions, and use the search function at the top of the page and you'll learn everything you need.