Is Ubuntu an absolute must use OS? Where are you trying to host this? Locally or on the cloud?
If you just want a free and easy way to try out a hosted version of ERPNext, have a look at the guide link below. It is not using Ubuntu (uses Debian 8 instead), but it does work and it comes online in less than 30 minutes. It also helps that it is free to try for up to a year!
Further down the thread in that same guide you can also learn how to make reusable and preloaded virtual images that can be made in just about 10 minutes.
Regardless, I believe it is probably the absolute fastest and cheapest way to make your own test system.
BKM