Looking for advice on quest creation

Hello,
I'm being stuck at game making.
I need some help with the organization/planning of how to procede.
I want to include quests in the game, but I'm not sure how to add them in a good (= less time consuming) way.

The quests should be selected by the player, so there is no order. I plan have at least 8-12 NPCs (as Non-Player-Characters in the story, not counting as objects or triggers) that will interact with the hero, giving hints or just changing positions according to what quest is active. (There will be lots of quests and I can't say how many yet.) There are ca. 15 maps up to now.
For example, the NPC's rooms, (where usually only one NPC can be met) or classroom where all of them could be met...

I was thinking about managing the NPCs and their behavior with plotscripts, so that I don't have to use the editor exsessively, wich would lead to me loosing overview soon.
I guess, I'd have to check for active quest's by tags (for what quest is active) and for the current map. After that a plotscript will create and move the NPCs, give them textboxes ect.

Do you think that is a good way of working? :-/
I'm not sure, and I really don't want to go halfway and than figure out that doing things diffrently would save me (work)time.

So, how would you go about quests and why?

I think one way to go about doing player chosen quests is to make a sub-menu where you can have menu items for each quest, that is enabled with tags (e.g. tag X being on means quest X is active) and removed when the quest is completed. Or maybe not removed, but changed so that it's marked as being complete. The player could open that quest in the Quest sub-menu and view the quest description, perhaps a checklist of quest objectives a la Skyrim.

Usage of tags would be the most straightforward way to approach a quest system, because it's easy as pie to have a text box conditional go to a quest-related text box for an NPC if that "tag X for quest X" is turned on, or having a text box that doesn't relate to that quest if tag X is off, either because the quest wasn't activated yet or it's already been finished. Then have other tags relating to smaller parts of a quest, but have a "Quest Active" on/off switch for whether the quest is active or not.

Yes, tags are the best way to do this. How I'd do it is I'd have a tag, let's say tag 5 be the no quest tag. So, when there's no quest selected tag 5 would be off, as would all the other quest tags.
Next, you would have a copy of each of the npc's you plan to have. Each copy would be for a different quest being active, or no quest. Then you would have the no quest npc show up only when tag 5 is off. The other npcs should be made to show up only when their quest's tag is on.
Now, you need a fancy custom menu. This custom menu should have a menu item for each quest. They should all be made to only be active when tag 5 is off, and when selected each menu item will turn tag 5 on and also turn it's quest tag on.
Finally, when a quest is complete you'll want to turn off tag 5, turn off the quest's tag, and turn on a quest complete tag. This quest complete tag should disable and hide the menu item for the quest and also affect any sort of change the quest being complete would affect.

That should just about do it.

These are good suggestions. One thing I would add, is that you don't have to worry about all your quests, just worry about making the first quest. When that is finished, you can worry about making the second one. Break the big job down into smaller jobs.