Where do I start with a home theater system?

If you can keep the HT separate from the two channel system all the better for your two channel. Then start from the beginning and go cheap. You don't need to spend a lot of money to enjoy home theater.

I suggest a new receiver with the most up to date codecs and room correction. 7.1 is better. You rarely, if at all, hear of anybody going back to 5.1. The receiver doesn't have to be top of the line. The 7.1 matrixing of 5.1 media works very well and creates a much bigger surround field even in a small room.

Its best to use seven matched speakers but using the matched center is fine. Stay small and thrifty with the speakers and hopefully used.

A good sub is what makes HT fun so budget more here. Again used, you'll have room correction in your receiver so you don't need a sub with correction software.

Unless your already a big movie renter I'd forgo the disc player until later.

Just my opinion, this is from previous experience.

Stick with Two channel!

You can try out home theater if you would like, everybody has their preference.

I went from Two Channel to home theater back in the 90s and promptly returned to the better sounding 2 channel and of course the constant upgrades to the latest processor.

But for some reason I thought that with th advent of HD, Blu-ray players and HD sound along with the fact that a lot of hi-end company's were producing surround sound processors, I would jump into the band wagon again. I went with a 7.1 system, after spend countless dollars buying large, full Range Speakers for all channels including a pair of floorstanding full range speakers for the center channel. Purchasing big Tube amps for All channels, including bi-amping the main speakers. Top of the line Blu-ray player and hi-end Processor and yadda yadda yadda.
Brands varied as I tried to tweak in the system ranged from, Von Schwiekert, Vandersteen, Acoustic Zen, Cary, McIntosh, Denon, Jolida, etc.

Yes, the system did play movies and to the laymen it was most likely awesome. But for me the surround processing varied so much film from film it made it hard to get things right with out constant adjusting of the settings. The processing also robbed the full range speakers from doing their job and would take information away from them, what I mean by this is if your listen to the movie in surround and then switched to stereo the sound would change. No matter what I tried I could never get things just right. So I went back to 2 channel and haven't regreded since.

So besides the cost factor of going multi channel, I don't think you really gain anything over a quality 2 channel set-up.

But if you want to go the home theater route, all I can suggest is that you buy a McIntosh Processor. I have not tried them all but I have tried out various budget models (Outlaw, Marantz, Rotel) and I have tried out some hi-end models (McIntosh, Cary, Theta). The Newer McIntosh Processors are quite expensive but as long as you can due with-out HDMI switching then I would recommend the MX-119. Wonderful sounding and probable the easiest to adjust. If the McIntosh is too much and you want to go budget, i liked the Outlaw Processors (no HDMI switching). If you have a nicer Blu-Ray player with 7.1 analog out you can still benefit from the HD sound.

Of course if you want have a dedicated theater room designed and built, i am sure your results would be better than mine.

Regardless of what you do I hope you enjoy the music and or movies!

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.