I recently started a project to install a 300E or 300TE engine into my 190E 8v, but it was too difficult to find a safe, reliable engine for sensible money. I am holding out for a 16v or 500E now, though neither would have the cache of the 3.0 straight six in a 190E.
I can only help you with the firing order - it should be written on top of the engine block somewhere.
If you could post pictures as you do this, or at least keep adding to this thread to keep the forum updated on your project, that would be excellent!
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black