If I had to compare DA:I to another game, I'd say it was like Marvel Ultimate Alliance... I doubt it'll be too challenging except on the highest difficulty level, but even then that can be mitigated by character level, equipment, tactics, etc...
Anyway, I'm pretty much in the same boat. I don't plan to play MP at all until I finish SP once so that I can get the full, uninterrupted experience. I figure that will be more than enough practice to do pretty well in MP... so long as I'm playing as a mage. Actually warrior might not be that bad either, but I'm going to stay away from rogues if possible... if I have to play as a rogue, it would need to be one of the archers... I simply do not trust being unable to block or defend myself outside of dodging. We only get a couple of potions to work with, and I'd rather stay on my feet until the boss is defeated...
Well, I'm going to play as a dual-wield rogue first thing simply because that's my favorite class in DA. Then again, Vanguard is my favorite class in ME, yet I rarely play as one in ME3 MP because of how buggy it can be off-host. I find myself playing Engineers on it more often, and that's my second favorite class.
The way I see the MP is being a way to try out builds without having to start a new playthrough in the SP. It allows you to see what works, what doesn't, what's the more fun to play, and what's the least fun. Then there are the characters that have abilities that are unique to them, which allows you to try tactics you wouldn't be able to try in SP. I will try every character in DA MP, but I'm going to start with Assassin simply because it's my favorite class and my favorite rogue spec.
As for when I'll play DA MP... I'm not sure. idk when I'll finish my first DA:I playthrough, and since this is the first game I'm getting on day 1 and it has MP capability, I would prefer to start fresh on the MP at the same time as everyone else. I'm definitely going straight into the story when DA:I launches, but whether I'll wait until I've finished the campaign before jumping onto the MP or if I'll play the campaign for a while before jumping onto the MP is something I'm still deciding on. It wasn't a problem before, but now that I'm thinking about it, I'm not sure which route I'll go.