Is it possible to run Snow Leopard (Server) inside VirtualBox on a Mac running Lion?
Yup. You should be able to. Give it a try. :) 

Though, I hope apps will be updated to work on Intel, because this isn't the most resource-firendly solution. It probably takes more processing power / RAM / Hard Disk Space to install and run Snow Leopard than to run the app itself. 

Yet, I think you can find alternative Intel apps. I really like using Apple's Mail for e-mail, and this article shows several alternatives to Quicken. 

Anyhow, its your choice. Give it a try and see what works better for you.