Better yet, are there any Spacewalk-like software that works for both RedHat-based and Debian-based systems?
There is. In addition to straight Puppet and Spacewalk, there is a puppet-based system called "foreman" that does most of what you're looking for.
while the screenshot does show CentOS and Windows clients, it does have Ubuntu support
I've used it and I recommend giving it a go.
According to the FAQ, Spacewalk supports Debian, so it may support Ubuntu as well. http://spacewalk.redhat.com/faq.html
If you are looking for configuration options for a large environment, you could take a look at Puppet http://puppetlabs.com/