As you said, if I go for Ubuntu. The existing erpnext v 13 I am hosting on virtual box VM with many modifications in doctypes, formats and few scripts. I also take regular backups from terminal as well as complete machine backup as *.ova.
But in case I install on Ubuntu, how would I restore or migrate my existing erpnext from VM to Ubuntu?
You could run virtualbox on linux as well, and just keep the .ova that you have now, but as Muzzy says, the performance will not be great.
You can also do a backup of the ERPNext subdir and copy it over to the ubuntu host, or you could do a bench backup with pvt/pub files, and then restore that to your newly created ERPNext installation on the desktop machine
With many options of cloud for inexpensive and even free VPS, I suggest use the cloud (FrappeCloud, AWS, GCP, DO, etc). You will avoid many headache with maintaining the server (because you also put #2 and #3 in your list).
With AWS you can apply for Founder/startup funding (free server) (I don’t know if it’s available in your area). In my case, this last about a year - with separate server for app and db.