Hey, welcome.
I can give you tips on the core DnD mechanics. I haven't progessed further than Act 1 in the game though, so plotting out details dealing with the game will be beyond me.
A) There are a few things I will point out with this one for you, and I will stick to the choices you mentioned of 2 Fighter levels, or Barbarian or Ranger as it applies to what I saw in the game:
1. 2 Fighter levels gives you: 2 bonus feats, all armor and shield proficiency, proficiency with martial weapons, +2 BAB (makes it easier to hit), +3 Fort saves and negligable skill points (2 + Intelligence Mod./level). All of this is useful to a rogue/assassin, except for the armor and shield proficiencies. Unless you are going for a brute that doesn't sneak around, and just runs behind enemies in the middle of combat to stab them in the back. Which, if that is the case, Assassin isn't the class for you at all.
2. 2 Barbarian levels will give you: +2d12 HP, +2 BAB, +3 Fort saves, Martial weapon and medium armor proficiency, Rage 1/day, Fast movement, decent skills (4+ Int Mod/level). The rest isn't useful, and the Uncanny dodge is a feature that you get both as a rogue and Assassin. To be honest, this may be your best bet. The higher speed, HP, BAB and decent skills are all useful. The Rage function will add damage and the ability to hit, which could make a difference if a fight turns ugly for you after sneaking into one.
3. 2 Ranger levels give you: +2d8 HP, +2 BAB, +3 Fort and Ref saves, Martial Weapon Proficiency, as well as a +2 bonus to many sense related skills and weapon damage against a specifically chosen Favored Enemy, the Track feat, and Either Two-weapon fighting or Rapid Shot, as well as good skills (6 + Int/level). This would be good if you were going for a stealthy character who used two weapons and didn't want your Hide and Move Silently skills to lag behind when you divert from rogue and Assassin.
To be honest, I wouldn't suggest playing a rogue build the first time around however. It takes patience, and is slow going as you sneak around. You would probaly enjoy doing so on a later run through more than the first time. Or at least I know I would.
Monk: It's a hard class to get right, same as a paladin. I wouldnt' suggest it for a first run through when you don't know the rules of DnD to be honest. However, some people like challenges. If you do, well then the above classes you picked would be good for you.
To be honest, if you want a decent build that should see you through just about anything, I have a suggestion. It is durable, can heal, buff and at later levels, dish out and take punishment like any fighter. A dwarven cleric with the Sun and Healing domains that goes into the Dwarven Defender class will give you a blend of the two most durable and battle ready classes there are in the game. You won't have as many feats to play with as a fighter, or as many spells as a cleric. However, you will be a self-sufficient wall with a weapon at later levels. It will free your party up to be able to snag another warrior, a rogue, and an arcane spellcaster for the party as well.
Yet, as I said, it seems as though you want to try something complicated from your choices up above. If so, I might be able to help you pinpoint what you think would be best for you if you went into more detail in what you wanted to do most with your character. The three builds you outlined are quite different from each other.
but im eager to learn!