MediaWiki is awful for images and files.
Awful isn't strong enough a word.
The WYSIWYG editor a forum uses is a lot better, in my experience.
Definitely.
Xencarta is the first usable wiki with WYSIWYG.
IPB has *sort of* a wiki system, but it's obvious their devs don't understand what a wiki is for, and don't consider it important.
I evaulated it. My impression is the same as yours.
This feature is absolutely critical for me
Critical ? You must be dying then. As I doubt you have a current solution that is good, as there are no good solutions. I've looked "for years". And looked. Definitely no other wikis are able to insert wiki content into a post.
and I would pay any amount of money I could afford to get it.
Any ?

I've added you to my "wiki list" (which is growing and growing).
The world needs a good wiki / documents engine.
There is a very significant untapped market here. All previous wikis are terrible.
And I know with a little work, a good wiki will be
absolute killer SEO. If you assign some people to keep the wiki up to date.
Give it 6 months and it will be
SEO GOLD. Gold Gold Gold. It's going to make forum posts look bad !

More importantly, a good COMMUNITY documents engine (wiki) is very useful.
I will never use any third-party mods or applications.
Never say never

It'll be a VERY long time before Xenforo releases a native wiki, if ever.
I have never had a single third-party mod with any forum software that didn't eventually stop working, contained serious bugs I discover after a month of usage, get abandoned, etc. I run a large commercial website and need someone I can hold responsible if my forum software doesn't work correctly.
#1) Jaxel uses his own wiki. That's important. His site is the best Xenforo site so far. He'll be updating his site well before you

#2) Any amount of money ? Jaxel's unemployed. I suspect a deal could be reached. The guy is online 24/7/365 with no signs of stopping. Support seems very easy. One reason why the current iteration of XenCarta won't break is there are very few dependencies on Xenforo (unlike vaultwiki). Try this: Pay him monthly via Paypal. If he slips up, don't pay him that month. Setup a fair plan. I think you'll be surprised at the level of support.
#3) This is a unique time, Xenforo is a unique platform, Jaxel is a unique programmer with unique experience in wikis. This unique opportunity needs to be seized !
More later. I gotta do some work

Xenforo + 2011 + WYSIWYG Community Wiki-based Documents engine (Jaxel) + key people = Awesome.
And I haven't even discussed all the killer killer wiki ideas. Can't wait.