Having WoG installed won't prevent you from playing regular HoMM3. For instance, if you want to play a random scenario without WoG, you will have to unselect all the WoG options in every tab (you can even save your settings if you want to).
As for campaigns, you will have choices between "Restoration of Erathia", "Armageddon's blade" and the WoG campaign.
Edit : anyway, you will have a WoG shortcut on your desktop. So if you don't want to bother with all the WoG options, just launch HoMM3 via your regular shortcut.

HoMM 3 complete comes with Shadow of Death, if you're buying it from GoG.

Is the current version of WoG at least stable? I remembere playing WoG 2 or 3 years ago and it kept crashing (the bigger map, the bigger chance to crash)

I don't know which version you tried, but I haven't ever had any problems with 3.58f, the so-far latest version.
(Hmm, until now; but even now it wasn't a crash, it froze up the system. It may be because of Windows 7, though.)

Just to know : for non-english windows users, WoG will require you to change your OS language and location to an english-speaking location (USA, GB, Australia, etc). But this is just during the installation. When it is installed, you can switch back your computer to the original settings. I did it as the game requested it and it works flawlessly.

Here's something not mentioned in the first post: WoG's random map generator is WAAAAY better than Heroes 3's original one. I've only been using that one for years.

I have played probably some 3.57 or 3.58 version, I can't remember which one was actual that time.
The error message was somethiung like:
ERM SCRIPT ERROR
<a bunch of scripts>
Press OK to continue
After clicking the game crashed to desktop.

NOW it produced this error for me as well (while I was playing this morning).
However, loading up the autosave from the end of the last turn solved the problem.

I considered giving this a go, but the horrible english on display in many of the screens turned me off. It's as if it was written by a bunch of japanese school kids.

Please reconsider. You're missing out on a lot of fun.

What did those horrible English texts read? Maybe you saw a couple of "moster mutterings", which are nonsense on purpose.
Personally, I don't recall any serious errors in the English text of WoG.

This is pretty horrendous:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/grm1.jpg
This is certainly not schoolbook english:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/broken4.jpg
This is indeed broken:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/broken3.jpg
Eyes bleed after two paragraphs:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/ivan2.jpg
And this could be taken right from a 1988 japanese-to-english NES translation:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/grm2.jpg
If monks not can you help, crazy, then go home, Kirr:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/ivan4.jpg
This is a long post and I have no time either:
http://wakeofthegods.strategyplanet.gamespy.com/img/wog/ivan5.jpg

It seems they posted some shots from their insider, "under construction" version; in the complete one, there are no such atrocities.
I'll post three shots which I've just taken to show the difference. They correspond to the second, third and fourth one, respectively.

Some info about new WoG 3.59, even with a video of a new, ancient Egypt-like town.
Also another mod, Horn of the Abyss, which is not yet finished, it will add some new towns (more info in the Celestial Heavens thread in the link).