Relatively new to the group so bare with me as I have some quick questions
So I have restored a SE30 which I assume is in stock condition with 8 Meg Ram etc.
You should immediately check whether it has been recapped. If not, consider doing it - the sooner the better. If you don't do it, you risk that the capacitors leak and literally corrode your logic board.
Also, check the age of the 1/2 AA battery. It has the potential to burst and spread acid all over the logic board and case.
My suggestion: If you don't plan to use the Mac on a regular basis, don't insert a battery at all. What you will miss is that contents of the PRAM are not secured, which basically means date/time and loudspeaker volume.
So what is the best / most stable OS version and where to source it?
From my point of view, a stable *and fast* version would be System 7.1. Back in the days when I started with these Macs, the version has not been publicly available (in contrast to 7.0.1 and 7.5.3). Meanwhile, most versions should be archived and available at such places like archive.org. I live in Germany, so I could just show you where to get German versions which most likely won't help you.
AFAIR, 7.1 is fast and uses less system resources. Another option would be System 7.5.3. It uses more resources and runs notably slower, but thenumber of available software is much bigger.
If you are going to install your system from scratch anyway, one thing you (c/sh)ould consider is replacing the original harddrive by a (SD-card-based) BlueSCSI board. Installing your system is much easier then because you don't have to create a bunch of installation disks and have the hassle of using 3,5" floppies with your modern system. There are also ready-made hard disk images for BlueSCSI around and the very handy "DiskJockey" tool, see here: https://github.com/erichelgeson/BlueSCSI/wiki/Getting-Started
Can I get the SE30 onto the internet and if so what is the best card / solution to do it?
To be honest: I don't know what could be a solution which would be *easy to acquire*.
What I can think of spontaneously are these options:
Use LocalTalk hardware and a LocalTalk to ethernet bridge. Has the disadvantage of being slow, AFAIR.
Next: Buy an external SCSI-based ethernet "card", a popular one would be Asanté EN/SC.
An additional option is to use the internal expansion slot of the SE/30 and install an internal ethernet card.
Here are some suggestions: https://se30forever.com/networking.html
Another option would be to just put a modem or modem emulator to the serial port, there are several options to bridge that into something modern and ethernet, but needs additional hardware and maybe things like running some server-like stuff on a Linux machine, etc.