Does anyone have any basic information / guide to installing mods for this game for starters?

I've noticed there are coop mods, bots and levels available in rune-world.com.

What do you recommend for coop?

I'm afraid I can't say much about coop mods, but one interesting mod I found so far is the Rune Extended mod, which puts ten levels that were removed from the campaign in Rune Classic back into the game.

I'm haven't checked rune-world.com yet, but you can look for a few mods on www.moddb.com. I don't know if they are compatible with the "Classic" release or not.
In the past I installed 2 mods, the coop mod (but I actually never tested it) and the bots mod, to play multiplayer modes against bots (it worked quite well).
And both had installers, so I didn't have to do anything special to install them.