So, I was a huge fan of MM 6, 7 and 8, but skipped 9 despite being very excited because of the panning reviews and with the company folding it just kind of felt like participating in their failure. However, it's currently $1.49, so why not? I know the GOG version is patched to 1.3, which includes the ELP patch. Does it include anything else?

What else should I download, modify, tweak, etc. to make it function like it should have?

I also saw a clue about meeting a messenger at some point and if you miss the meeting, the main quest breaks. I also saw that some high level spells simply don't work and the ELP couldn't fix them.

The game is still playable if somewhat infuriating at times. It definitely has a nastier learning curve than 6, 7, and 8. But the basics still apply: bows are good, so are buff spells. Early on you have to do a bit of grinding to develop experience before you can approach some areas that no go zones for low level parties. What helps is those initial areas repopulate quickly. You will develop a taste for killing Thralls after all. I tend to prefer magic heavy parties and spell and development system might take some getting used to.

As for mods and such can't help you there I have only played it out of the box from GOG and of course from the original CDs back in the day.

Flamestryke's MM9 website will surely help with playing it.

https://www.zimlab.com/wizardry/recovered/flamestryke/mm9/flamestrykes_mm9.html