1. Am I right in thinking that no matter how powerful my character, I will start at the beginning with ME2 (possibly with a slight bonus for getting to level 50 or 60)?
2. Should I just spend some of my points on Charm to get it to a high level (and how high) and accept that I don't have the most powerful character possible?
3. Should I spend any points on Intimidate, or are they pointless for a character that's playing as mostly a Paragon?
4. I've not yet had an option for Shock Trooper or Nemesis.
1. Level 50+ imports into ME2 will start your character at level 3 with some credit bonuses. Level 60 import will start you at level 5 and a few more credits. Anything less than level 50 will start you at level 1 or 2 (I forget). You can get additional credit bonuses for getting the Rich achievement in ME1 (get 1 000 000 credits in ME1).
Starting at level 5 (level 60 import from ME1) is better than starting at level 3 (level 50-59 import from ME1), but it is not that critical. You also have to keep in mind that to get to level 60 requires at least two full playthroughs in ME1. If you do everything both times, you will probably spend about 20-40 hours each playthrough, so 40-80 hours for two playthroughs. It can be a slog sometimes, so just using the one playthrough is good enough to get what you want.
I would play through twice to get to level 60 to get the Extreme Power Gamer achievement, but that is about it.
2. Spending points on Charm/Intimidate trees depends on whether you want all persuade options in the game. The highest Charm requirement is 12. The highest Intimidate requirement is 10, unless you happen to miss one specific opportunity with Saren in the middle of the game, and then the final check requires 12 Intimidate (otherwise it would only require 9).
Charm/Intimidate are used only for specific dialogues with certain NPCs on quests, typically to get you to avoid a fight or to keep people alive. (Using Charm/Intimidate to avoid a fight will net you the same XP as you would get if you fought.)
You get up to four free Charm/Intimidate points as you play the game: 1 Charm and 1 Intimidate for becoming a Spectre, and 1 point at 10%, 25%, and 75% Charm/intimidate. So to max your skill tree you need to spend 8 points and pick mostly Paragon or Renegade responses most of the game; Charm and Intimidate will get you extra paragon and renegade points, and some actions will give you charm or intimidate points.
Maxing Charm/intimidate means you have 8 less points to spend on combat powers, so basically you will not get one or two powers to Master. Not that big of deal really. With Vanguard using Singularity you are already going to have to not max a few powers, but Advanced is not too bad and with Adrenaline Burst you can reset your cooldowns and cast again so it is not too bad anyway.
Not maxing Charm/Intimidate means some NPCs will die and they will not be mentioned or appear in ME2 or ME3. Not that big of a deal, but it is kind of neat to see them or hear from them or hear about them.
If you play ME1 more than once, you can max Charm/Intimidate with one character and then not put any points into them with another character and see the difference as you play through the trilogy.
3. Do not put any points into Intimidate if you are playing mostly Paragon. Putting points into both Charm and Intimidate is discouraged unless you do not mind having weaker combat powers. I have done it before, but my combat was sub-optimal, although by the end of the game with good equipment you probably will not notice.
4. When you reach level 20, the next time you access the galaxy map and travel to a new system, Admiral Hackett will contact you and give you the Rogue VI on Luna mission. (NOTE: sometimes another call will take precedence depending on the system you enter. I recommend going to the Widow Nebula where the Citadel is located, or another system you already visited. You may need to exit the galaxy map and then go back and travel again.)
Completing this mission will give you access to your choice of specialization: for the Vanguard your choice is Nemesis or Shock Trooper. Take Shock Trooper.
I also recommend this thread: [Mass Effect 1] Start of the Journey: Single Playthrough Builds by Simbacca. It is a fantastic resource for single playthrough level 50 builds.