How much space you need depends on the size of your project and what you will be keeping under revision control. If you are storing large binary files then the size of your repo could balloon. That being said, in most cases your repo shouldn’t take up a lot of space.
All you really need to get started with SVN is SVN. If you want to access with svnserve you will need to fire the daemon, but if you use svn+ssh (which you should), you can leave the daemon off, as it will be started automatically on each request. If you want to set up webdav, you will need to make sure Apache is set up and should have mod_dav_svn and mod_authz_svn installed. If you are using another web server, you should check out the manual to see what needs to be done. I know for a fact that the Nginx webdav module isn’t really ready to roll, so if you use Nginx and want svn over webdav you will need to proxy to Apache. I don’t know about Lighttpd.
As far as your repo/project structure, that is up to you. It is easier to work with a single repository with multiple projects, but if a lot of different users need access, you will have to configure all of the acl stuff which could quickly become a pita. Multiple repos are fine too, but it just means more overhead when doing things like backups and sharing code between projects. Your choice.