If this site is static, you can run the index.html and those other two pages, install MODX right along side it in the root, develop and when you're ready just rename index.html and turn FURL's on.
This method works nicely, I've got a couple sites running MODX in development mode along side static html files. Insofar as the installation, it can be tricky at first but once you know the steps well it's very straightforward. I think it's good to know how to install MODX manually. Some of the script installers that are provided by your server are good, and some aren't. It largely depends on your hosting's configuration although it may be worth a try installing it that way if you have the option to do so.
Who are you hosting with? Seems to me that given the time you'll spend wiggling with the php form, we could instead help you get MODX installed so that you can then get the site up with a working contact form.