How to re-install OS on Mac 8.1

I am trying to sort out a Mac 8.1 for my sister and, although i am computer literate, I am more familiar with Windows than Apple stuff and it is proving to be a struggle!

Basically, the Mac was set up by her ex-husband and still has a load of his stuff on. I would like to reset it to a nice, clean machine that she can use and figure the best way is to wipe the disk and re-install the operating system.

A lot of guides refer to Recovery Mode, but, it seems that this is not available on this model.

Others suggest using the original installation disk, but, this is not available and I am wondering what I can do.

The current version is 10.5.8.

I see that there are some disks being sold on eBay - not necessarily this version but things like Snow Leopard, Lion, Yosemite, El Capitan which, according to a website I looked at, should be compatible.

First question.....should I be trying to install exactly the same version or is it reasonable to go for a later version?

Second question......should I be OK buying one of these Installation Disks and be able to use it to install or do I also need a licence key?

Third question.......is there any way to avoid buying the disk and is it possible to download an Operating System, write it to a DVD/USB stick and use to install?

Sorry there are so many questions, but, I am trying to feel my way through a unknown technology!