Am I correct in thinking that all I really need for a backup , as long as I haven't made any custom changes, is a backup of the database and a copy of my upload folder?
Then if needed, I can reinstall Serendipity and restore the database. Then replace the upload folder with my saved one.
backup
Yes, you are basically correct. Backing up those locations will allow you to get back to your original site with a little bit of work.
I would also backup the templates directory if you are not using the default template. Same with the plugins folder. Restoring these directories will allow you to restore your site easily. If you use anything not on a standard install then you will get errors when trying to restore due to templates or plugins needing to be install.
In my experience backing up the entire s9y folder and the database is the best solution. This way you can get back up and running with the least amount of work.
I would also backup the templates directory if you are not using the default template. Same with the plugins folder. Restoring these directories will allow you to restore your site easily. If you use anything not on a standard install then you will get errors when trying to restore due to templates or plugins needing to be install.
In my experience backing up the entire s9y folder and the database is the best solution. This way you can get back up and running with the least amount of work.