1. Firstly you'll want to make sure that JobRoller offers the features you're looking for in a theme/for your site, you can find this information on the main
JobRoller page. You can certainly put the site in maintenance mode by way of a plugin if you wish to work on it before making it public.
2. Yes, web hosting/domain names are separate to the JobRoller purchase, and you need to have an instance of WordPress installed in order to use JobRoller. On a related note, you can read about our WordPress Hosting promotion
here.
3. No it does not.
4. If out of the box, JobRoller gives you the features and layout you're happy with then yes, however this is in a very general sense given that I know nothing about your site. Thanks.