Hardware and performance

There are quite a few excellent suggestions here, but I think you're using the wrong tool for the job. While for Moodle it would be easier to write authentication modules and other customizations, why not go with survey software instead as that doesn't have all the things that slow down Moodle (and make it useful)?

I'd suggest looking at something like "LimeSurvey" as I believe it should meet your requirements and it stores very few things in the database so you don't have to invest in more than one database server.