For your needs, a baseline 15" MacBook Pro would be sufficient, I think. The only upgrade I would recommend is to increase the drive size to at least 512GB (the more space, the better). I don't have Omnisphere, but I've heard the storage required for it is pretty large (~50GB), so between a full Logic instal and that one plugin, you'd have 100GB + taken up already. I can't speak on Trillian or EZ Drummer, either, but I imagine they take a little chunk as well. The CPU, being a 2.2GHz 6-Core i7 will give you plenty of power, and 16GB of RAM would be more than adequate as well. All subject to change if you have really large sessions, but that was indicated as much of a factor.
For an interface, I'd recommend something from Apogee since you'll have plenty of money left in your budget. The new Element series is a nice, affordable option (same converters as the Ensemble, IIRC). If by offline processing, you mean handling the DSP for plugins, UAD makes good interfaces as well that will do this. You'd need to be using their plugins, though, for that to apply.