Where do I start with a home theater system?

Based on your current system, IMHO, I would get a center channel compatible with your planars (maggie?); a 3 channel power amp with similar characteristics similar to the levinson (parasound, classe or Levinson if they make one) a cheap processer ( the Emotiva umc-1 is pretty good for $500 and you can "pass thru the signals from your 2 ch preamp thus keeping your 2 ch listening in tact. Then I would get the OPPO 95 or 105? for your surround sound player and use the analog ins to your processer (quality will be better than any costing $2-3K)
And of course , subwoofers. This would give you a great surround setup while still enjoying your 2 ch.

PS I have a surround sound using Apogee speakers with good electronics--stunning!

If you go 7.1 as Vicdamone advises, make sure your room can accommodate all the speakers, especially the back surrounds.

Finsup, I have had very satisfying results setting up 7.1 in small rooms. In small rooms were the listening position is backed up to the rear wall I installing non dipole speakers behind and above the listening position. Pointed them directly at the ceiling and letting the room correction function works very good for the rear effect.

The side surrounds can also be mounted near the ceiling aimed at the listening position.

I agree with Madhf.

I have incorporated a center channel, a 3-channel amp to drive center and rear speakers, a pretty good processor(Integra) and an Oppo BDP-95 for movies only. I integrated a sub I already had and run the processed signal through my 2-channel preamp and amps for the front 2 channels. After a little balancing and room correction, I have excellent movie sound when I want it without any deleterious effect on my 2-channel system whatsoever. Its a win-win and what I would recommend if you can pull it off.