Adding Home Theater To 2 Channel System

I'm in the two separate system camp
In my credenza l have my VAC tube amp/ARC tube pre amp and my Emotiva XMC-2 16 channel!! Processor & Anthem MCA-5 that I've owned forever  I have two set of main speakers, center speaker and two SVS Sub woofers My rear speakers are wireless  l just turn on the system that l want to listen too 

You can do without a center channel but HT is carefully developed to have dialog specifically coming from a center. Because the frequencies sent to the center channel are relatively easy to produce so you can go with a very different design of speaker for center.
And quality or price is not critical-within reason.
A very interesting choice would be the MBL 111 single speaker currently for sale on AG. I think that is the model #. It is the small entry level one. It has been for sale for quite a while for about $2500.
Surely he should take a decent offer. That would be a super interesting center to combine with your Unos!
A completely separate system except for the Unos is best.
I don’t think it would be a problem to run additional speaker cables so that the Unos would have two pair of cables each.
Certainly, your choice of TH components depends on your wallet.
I have seen HT done with tubes but SS is most common.
Most important is to have the same amp for the RL and C. Best is a 5 channel amp and a good processor. What I would get is an older top line Meridian processor. You can pick one up for a fraction of their original price. Features will be more than you will need.
And getting an older pair of Meridian powered speakers would be great. Then you will need a 3 channel amp or 3 mono blocks.
I would get (unless you already have on) an OLED TV. Sony is best.
At least 65” but I would get a 75”. Going with projector and screen is tricky and probably double or more. A projector will give you about a 100” picture but not necessarily a sharper image.
In any HT, the processor is the most important and what you build your system around. For SQ and functionality, I love the Meridian. Especially since the older ones are still perfect for a basic 5.2 system. Think your subs can also take double cables but I think a Meridian also has two rca sub outs. I just want to emphasize that an older processor’s features will be all you need and more. The top line one is great but a G65 one can be had for about $1K (think the MSRP was over $5K!). It is also excellent. Mostly less features that you will not need.

HT is also designed to be backwards compatible. That is just how it works. 

Please don’t 

I have done this quite successfully.  I'm not saying this is the only way, it is just the way I accomplished it.

First, I agree with the statement that a centre speaker can mess things up. I tested a decent one (Vandersteen) and it added absolutely nothing to what my main speakers could do as a phantom centre.  Do your own test, though. Some main speakers don't do as well at creating that phantom image.

I picked up a decent AV preamp and a separate 5 channel power amp for the surrounds. The only commonality between my audio and video systems are that the main speakers are used for both purposes. 

If I want to switch from audio to video, all I have to do is  pull the speaker leads for the main speakers out of the one power amp and plug them into the other power amp, switch on the appropriate preamp and go.

I also have a pair of powered subs that are used only for video use.

It may seem like a lot of fuss, but it is the only way to accomplish really good sound in the audio system for me. There may be an AV preamp out there that sounds as good as my audio preamp/amp, but I haven't found one, and my audio system power amp at 70 wpc wouldn't run at video volumes easily.

Basically start with a good audio system and add on the pre/power and surrounds as a stand alone that takes about two minutes to switch to..