Kevin,
Let me try to answer your questions:
1) About 15 to 20 MB of space is used for the default install. That's less than 5% of your space.
2) Because of the independence of each VDS, you can install your own apps without any threat to any other VDS. However, we can not provide any support in doing so.
3) System resources are allocated dynamically, no single VDS has X memory and X processor time allocated to it. Also though, no single account can use all of the memory/processor at any given time.
4) If you manage the DNS externally, you could point another domain at your VDS. We don't have a mechanism to charge for that. You wouldn't be able to do it from within the VDS and Apache.
Hope this helps,