I'm pretty sure you can get the expensive motifs in-game.
Haven't gotten there yet, so I can't say for sure.

Right now, there isn't a whole lot on the store that's worth buying, imo.
I'd say save your crowns for some limited time mount (or pet, if ZOS ever does this, too).
But, I am more into collecting mounts and pets than much else.

Imperial City will cost you 2500 if you're buying it. The rest - mount lessons save you a hell of a lot of time, I paid for majority of my speed via the store.

The Imperial Edition and the Imperial City DLC are two completely different things.

The Imperial Edition is basically the game's special edition - it lets you make Imperial characters (who can be in any alliance), lets you convert most weapons and armor to the imperial style and gives you a white horse mount and a crab pet. Also Imperial characters can craft in the Imperial style - this is rarer than the other racial motifs and so harder to find and more expensive to buy.

The Imperial City isn't released yet so you can't buy it. But it's going to be a new PvP zone (with PvE activities like Cyrodiil has currently), 2 new dungeons and some new mechanics and items. You can read a lot about what will be included on the main site.

I wouldn't suggest getting the motif packs until you know more about the game and what you like to do in it. Unless you want to craft items yourself they're completely useless to you, and they only allow you to unlock styles on one character so you need to be completely sure they're the one you want before you use them.

Personally what I'd get (if I didn't already get it as a pre-order bonus) is the Adventurer's Pack for the Any Race in Any Alliance upgrade. I really like having the ability to choose which alliance my characters are in separate from their race. Even when I choose to keep them in the default one I like knowing it's my decision.

Also remember that you don't need to spend all your crowns now. They don't expire and there's new things added to the crown store on a regular basis so you can keep them until there's something you know you want.

As an obsessed crafter with no purple motifs yet, is buying the 5k rare motif pack (think it contains elf, barbaric, daedric, and primal) a pretty good idea?

If I buy them in-game its looking like it would cost between 150-200k, so it seems like it would be best value vs. imperial or dwemer which could be picked up for around 75-85k each.

Since my horse training is already advanced a good bit, and I'm not really into collecting pets or mounts, I really don't see a whole lot thats a good value in the store.