Setting up a system - What to buy WHEN?

I've built at least 2 dozen systems from ground up & I've always started with the speaker system , a solid 80% if not 90% of your initial budget should be to purchase a set of " full range " floor standing speakers that go down to at least 30hz & 20hz js even better ,make sure you get speakers that your absolutely in love with the sound and the looks because with a big ass speaker system asthetics become very important in your home ,you already have a good quality " HIFI " preamplifier with your $700 Rotel ,keeping the Rotel and building the system with the Rotel as your preamp will allow you to spend your next money on amification ,while amplification is important you do not need to spend over $300 on a pair of professional amplifiers that will give you 500 watts of 8ohm low distortion power if you visit your local guitar center & buy a pair of Behringer A-500 amps ,they are roughly $150 each and when ran in bridged mono give out 500 watts @8 ohms which is enough power to drive any speaker system with a DB rating above 88 db which is a very inefficient speaker and the amps will drive them ,I've done it , for the digital sources I'd visit Emotiva dot com and buy a brand new Emotiva ERC-3 CD player at $400 or less and you'd have a good quality CD player ,I own an Emotiva ERC-2 CD and it sounds just as good as my $5,000 McIntosh CD player , I'd scroll down Emotivas products line and buy a pair of top quality 12 foot speaker wires with banana plug terminations for $120 ,then for power conditioning I'd buy the Emotiva EMX-2 line restoration and power filter unit at $159 for my amplifiers and the Emotiva EMX-6 power filter for my preamp and sources for another $159 , I use the EMX-2s on my 1200 watt McIntosh monoblocks and the EMX-6 on my McIntosh tube preamp and McIntosh CD/DVD player with excellent results , I'd stay with that system for at least a year before I upgraded anything ,then I'd upgrade the Rotel preamp with a used McIntosh C-2200 tube preamp of which I have owned 1 for a decade , the McIntosh C-2200 is a very unique preamp that is the best I've ever heard ,the preamp has the most effect on overall sound than any other components except for your speakers ,once you've upgraded your preamp to a McIntosh C-2200 tube preamp as long as you chose the speakers you love it only gets better & you may not need to spend big bucks on high end amplifiers and sources , if your dead set on upgrading to better amplification Emotiva made a unicorn when they made their XPR-1 1,000 watt @8 ohm Monoblocks you can pick up used for $3,000 fairly easy & they compete well with my $26,000 McIntosh monoblocks .

Speakers 1st ,amplification 2nd ,source 3rd,power conditioning & quality speaker wires and interconnects 4th , upgrade your preamp if you must 5th ,then upgrade your amps last if the 500 watt Behringers aren't driving your speakers well enough .

Agree with @inna in that I would start with speakers. Speaker choice will dictate the requisite type of amp that will mate with them properly. A satisfactory experience relies on understanding the codependent relationship that speakers/amps necessarily have with each other. A preamp must also pair with the amp in terms of its gain structure and ability to interface with the amp impedance wise in a copascetic(?) manner.
Everything else will be less finicky as regards to its "fitting in" with the aforementioned items.

I personally favor high sensitivity speakers as these typically open up more opportunities to experience the uniquely seductive amplifiers out there and they are perfectly at home with lower power (read less expensive). Many other would take a different path.

Agree with others start with a speaker you like.

Yes, start with speakers, yet at the same time keep in mind the type of amp you will pair with it. 
Try to audition and see what amp works well with each speaker.

I disagree with the approach of buying components for the short term, then upgrading them after a year.
  Buying used will get you more bang for the buck. The result is a higher-end component for your allotted budget.

bigslacker

My plan is to hopefully be able to buy an amp that'll power both mains on it's own. Then circle back later, buy another of the same unit. Then I'll swap each over to monoblock mode - one driving each main.

It's really always a compromise to use bridged amplifiers, which is what it sounds like you're planning. A bridged amp is limited into low impedance loads.