While reading a review for Gothic 2 I red that the game is extremely challenging when the Night of the Raven is installed. Is this true? Should I not install NoTR if possible? Also it said that saves would be erased if you installed NoTR so I wouldn't be able to play NoTR with my character from the main game if I hadn;t installed it initially. Is that true?

I didn't spot this thread before answering to the "Gamepad" thread, but I "accidentally" answered it there as well :).
In short - yes - you should avoid the expansion on your first play-through. If you haven't played a Gothic game before it will most likely eat you alive. As I said elsewhere - it was meant for the veterans of the series who know all little details and tricks already. The reason why you cannot use your character from original G2 in expansion is because add-on is incorporated into the game, and it's content can't be separated from the original game - it is an add-in, so to speak. The characters you meet in G2 often have extended dialogue options and some additional quests in NotR.
But if you like original G2, then you will probably also like NotR, so you can play again, but this time use a different path (there are 3 factions to choose from), and see the side-quests that you have to miss on your first play-through due to your faction choice.

Paradoks is right. The beginning of the game for someone who is not familiar with it is about as pleasant as pulling teeth. You should play it without NoTR first and then try it again but join the other faction

Or play Gothic then Gothic 2/NotR. :P

NotR only adds quests to the first three chapters, so one option would be to only finish the expansion content. It's only from Chapter 3 onwards that it becomes insanely difficult all the time, so this would work fairly well.

I just finished Gothic I and I want to buy Gothic II, however, I have heard the difficulty from the add on for the second game makes the game much harder. I would like to know, as a person who has completed Risen (the spiritual successor of the Gothic games) and Gothic I, should I go ahead and play Gothic II with the add on? I'd be grateful if anyone answers this question.

Night of the Raven is significantly harder than any of the other games (with the possible exception of Risen on hard), but if you're up for a challenge it's certainly a rewarding experience.
Note that the GOG installer installs the add-on by default but you can change this in the installer settings. Playing through the original Gothic II would give you a better grounding in what to do when replaying with the expansion on a later occasion.

Well I find Risen a lot harder then Gothic 2 NotR. If you've completed Risen on default difficulty you shouldn't have any problems with G2 NotR.

NOTR is significantly more difficult, but there are some changes that make it so it's not unfair.
If I were you I would play Gothic 2 without the expansion first, since for someone that is not familiar with this series, it might be challenging. Once you've finished, then install NOTR and try a different path.
Also, in this thread you can read some of the tips for new players: http://www.gog.com/en/forum/gothic_series/gothic_2_new_player_tips_and_help.
The differences between Gothic 2 and NOTR are:
-enemies are a lot tougher
-spells cast from scrolls cost a fixed amount of mana
-there are a lot more enemies each chapter
-misc skills (skinning animals, brewing potions, etc.) have a varying skill point cost, instead of being fixed
-there's a new map to explore with more characters and quests...
-...and also new and old enemies (from Gothic 1)
-expanded roles for some of the secondary characters
-more quests and storylines related to Gothic 1
-acrobatics skill
All in all it's a great expansion but in my opinion it's better to play it first only if you have played Gothic 1.

You guys make it sound impossible.
NOTR is hard until u get to the new map Jharkendar.
My advice to you (I played NOTR twice) is to go for the warrior route. Get some strenght and invest in Two-handed swords. Chapter 1 will be the most difficult because you're weak but if you have skill you can beat pretty much everything 1v1. Join the militia or the mercenaries (preferably the militia) and do as many quests as possible (you can leave the harder ones for later). You should be about level 9 or 10 when you enter Chapter 2 (maybe lower/higher, can't remember exactly)
Once you get to Chapter 2 don't go the Valley of Mines. If you do that you'll be eaten alive by the Orcs (which are mugh tougher than in Gothic 2), instead go to the temple close to the Black Troll. Complete the water mage quests and entre use the portal to get to Jharkendar.
From there start doing quests, you'll find that many creatures are pretty weak and the harder quests can be done with the help of NPCs.
I don't wanna spoil the story but the point is to get the Claw of Beliar. You get it after you finish the main quest. That sword has a chance of hitting your opponents with lighting damage (the change increases with strenght I believe). It's a 2-handed sword and you'll pretty much devastate everything on the map with it. You can beat Elite Orcs with 5-6 hits. As long as you don't fight more than 2 opponents at once it's a piece of cake.
So NOTR is much easier than Gothic 2 from chapter 2 onward.
I recommend you buy it, it's a great experience.
I'm a RPG fanatic, I played BG, NWN, TES 3 + 4, The Witcher, Mass Effect 1+2, Diablo 1+2, Risen, Gothic 3, Dragon Age and many more. Gothic 1 and 2 (NOTR version) are still my favorites.