1) You can use plugins to manage your banner ads. ClassiPress itself has built in features to collect money for ads placed, but not actual banner ads.
2) You can create ad packages to accomodate free and paid ads, or charge per category. When you say "Main headings", if you are referring to categories, then these are all created by you, so you can choose whatever you want.
3) Yes you could certainly have all ads free to start with and then set up ad packages, or prices later. Note though, that if you set your ClassiPress admin options to allow for ad renewal, then those ads placed ads in that first month could technically be renewed for free.. so you'd probably want to turn off ad renewal at that time.
4) Yes, the Personal edition licence will allow you to collect payments via PayPal, but it doesn't include transaction logging or PayPal IPN integration, which means you can't view revenue information from your ClassiPress admin dashboard.
5) Adding a forum is a completely separate thing, and not built into ClassiPress but there is nothing stopping you from doing so by way of perhaps a subdomain link from Classipress. There are forum plugins but I cannot confirm if they are 100% compatible.
6) Your pricing options are Fixed price per ad, price per category or % of sellers ad price.
7) Yes.
8) Yes.
9) Not at this time, however there will be further enhancements to ClassiPress in the upcoming release (
version 3.1), and you may wish to wait for this to be released to confirm if any of the new features will suit your needs.