Well, several questions actually.
1) If you have the Raven expansion installed, do you play both the original game and the expansion at the same time? In other words, do you play the original all the way through, and then start the expansion? Or if you start the expansion first, do you play the original storyline at the same time?
2) How much more difficult is the expansion than the original?
I would like to play the expansion, but I don't want to have to play through the whole game twice (the whole original, then start the expansion).
Any and all assistance is greatly appreciated.
Regards,
Michael

You play the original and the expansion at the same time. If you play the original game without it you will miss out on a lot of content from the beginning till end. Let's just say some pirates are involved here and there....
If you install the expansion you will the game to be far more challenging than the already challenging original game. There will be no handholding, no level scaling, and, most definitely, no mercy from the game to the player. If you happen to wander into an area that you are not quite ready to be in, then the game will simply hand you your fundament on a finely crafted silver platter.

You have to play both at the same time. Basically, if memory serves, you should go until Chapter 3 or 4 of the main game "normally", then play and finish NotR, then finish the main game.
The NotR map is easier than the "normal world" map, and MUCH, MUCH ooohhh sooo MUCH easier than the former colony map or the "normal world" map after (SPOILER). When playing normally becomes too difficult, switch to NoTR, finish the quests, then come back.

Actually, I would suggest tackling Night of the Raven in Chapter 2 if possible. If you have built your character correctly you should not have too trouble doing this (certainly no more than when tackling orcs and other very tough creatures). You can probably gain about 10 levels in Jharkendar if you do all the quests available and kill any monsters that get in your way as you go from place to place, and this power boost will make the rest of Chapter 2 a lot easier. Jharkendar also has a large quantity of skill-boosting tablets which become immensely useful for boosting secondary skills or once the costs for your primary skills start getting too high, and you can also acquire some powerful weapons and magic items there if you explore thoroughly.

The tablets and stat boosting potions are the most important things in the expansion.
I finshed the game with NOTR twice. Once with strenght based melee fighter, and second time with dex based archer.
In first case a gave only 64 points in stenght and 34 in dexterity.
Tablets and potions took care of the rest.
Here are a few important maps:
http://www.worldofgothic.com/gothic2/?go=g2addonkarten
The most important are herbs and tablets. I collected all of them and at the end of the game I slayed everything that moves without breaking a sweat. You can also find dex and strenght potions and hrebs in few merchants inventory at the begininng of each act. If you want the list I can provide you wtih one.
I recommend finishing Jarkhendar at the begining of the act 2 just like Arkose said.
TOP TIP:
If you buy stat points for your learning points you will notice thet you have to spend more LP's past 30, 60, 90 and so on. But if you buy 5 points when your stat is at 29 you will still pay less learning points and have 34. You can save many learning points with this metod.
Few things about characters. I never play mages so I can't tell you anything about them. Melee warior is a lot more challenging at the and of the game then archer. With the latter you will struggle at the begining but at the and you will dispatch mobs of orcs without any problem. The choice is yours. Enjoy.