1. There is no current function built within the theme to allow multiple currencies to be used, so this would require customization on your part. You can however set one currency symbol from the front end and collect payments in another currency. You may wish to check out the
ClassiPress admin demo to see what it is you can set. No, there is no ready to use code for this mod, and on that note, you should be aware that customizations and third party plugins are not officially supported.
2. Yes, you can remove the "from the blog" and "twitter updates" widgets if you wish, and customise the sidebar with the widgets you want to use. I know of many customers who are successfully using 125x125 ad space widgets for their sidebar to handle outside advertising banners. Whether or not customizing the theme is difficult, really depends on your level of knowledge. I have seen customers who have created a sidebar on the left hand side, so it can be done.
3. The only PayPal integration used in ClassiPress is for collecting money from people for posting ads, not for customer transactions between each other. How they collect payment is up to them, as ClassiPress simply allows the means to make contact by using the contact form in the ad sidebar.
4. This is a customization question which we really can't confirm, as it's not part of the theme. Whether or not you could get something like this working is really up to you.
ClassiPress and JobRoller are both stand alone WordPress themes, not plugins, so yes, once activated they will each replace any existing WordPress theme you may be currently using.
I hope this info helps :)