Hey there Ronan
Wow it feels like I have stepped into a time capsule :grin:
He was wondering if it was possible to install it on a modern PC
This should be possible, I do not see why not. The only issue he may have would be that the OS doesn't like the big sized hard drive.
Also he was wondering where to get it from, he mentioned eBay
The only place I can think of is ebay unless you have any local computer stores which may have a copy lying around that they will sell you. (or give you

)
Is it easy to install? From CD or floppy?
Yes it is very easy to install, and the installation will be on floppy disks.
But if you are thinking of installing windows 3.1 you will need to have a version of DOS installed first. e.g DOS 6.2
First you install DOS from the floppy's and then you will install windows 3.1.
So you need to hunt around for both DOS and Windows disks.