Alright, a question!

I have run linux on a couple of macs and it has always been a pain in the ass, for one reason or the other. They definitely do not make it easy for you. I think it is possible to make work, but you might be better off just grabbing another hard drive and using that .. keep your Mac drive "just in case". You could also try running a Live USB version of linux .. and then when you're comfortable enough, nuke the Mac drive. If you are going to play around with this stuff a second bootable drive is always nice to have around. You might be able to just boot off an external drive, but I don't know if they'll let you do that. I assume so. I Do not have good experience with later model macs, and my older ones, I always wiped the drives and just ran OpenBSD.

For a VPN provider, I like Mullvad, though I have heard good things about Nord too: they are definitely one of the more respected ones. Mullvad is maybe a little more privacy/security focused. I have been very happy with them. Nord would be on my short list though.

For email, I can't suggest anything.. I don't particularly trust protonmail. My email is done by some people I know personally, so I feel pretty safe with them. If I didn't have that, I'd definitely roll my own, probably using squirrelmail as a client (over a vpn.) I wish I had a better answer here, but it comes down to trust, and .. trust no one!

As far as applications and whatnot, yeah, there are LOADS.. except for gaming, and maybe some niche software, you'll find everything you need. Do you need anything strange?

On the ibook, YES, it would probably make a great little server. You will want to find a minimalistic Linux or BSD though, as that hardware is going to be very taxed trying to run any of the larger linuxes. Puppy Linux might be an option, and if you are feeling brave, OpenBSD. If it were me, that is where I would start.. OpenBSD on the old iBook.. see how it goes .. see if you can make the environment work. And if that works well, do it on your main machine. That is probably a 32 bit processor, eh? Definitely check that, and make sure you get a suitable distribution. I think a lot of the linuxes nuked their 32bit trees recently, though some are still available.

Let me know what else you need to know - happy to help.