What a cool thing to do together! so happy that your wife is ok!
I would consider a big canister over a sump. (sumps are not c02 friendly...too much shakey shakey)
I put a fluval fx5 on my 125, not sure if an fx 6 would be enough on a 220 or not....
This is a false statement. I assume either you never tried it or you didn't do it correctly. As mentioned above, many have and are having success with it, myself included. CO2 issues with sumps were a thing of the past. Its been figured out how to make a setup with a sump work.
+1 on a sump, better lighting and 20lb co2. For a tank that size, its a win win. I won't go into detail on them because there are loads of threads out there under each given topic. Not just on this site but on the web in general. If you are a diy-er, build a stand if you don't have a good one. Tailor it to your liking, intents and space requirements. Place the largest tank you possibly can underneath for your sump.
There are many many pros to doing a sump. I will give you my top 3 but there are many others out there. 1) display maintains consistent water level- the sump evaporates, not the display 2) surface skimmer - gets rid of that unwanted protein layer and gives good gas exchange 3) increased water volume - more water volume usually creates less issues as parameters do not change as quickly as small tanks. A bonus is place to house your equipment which gives you a cleaner looking display. The only con really is that you need more space and you have more plumbing to do. You were a reefer so I am sure this is nothing new to you.
Substrate wise, I would probably go with something better than Eco though. There are better cheaper options. Depending on your intents for this tank will determine all the options available for you. Are you going for something simple, AGA contest style tank, a little bit of both, etc...... I think your on the right path for experimenting first. I have been working on my project since summer of 2012 and I am just starting to setup tanks. Take your time, be patient and research. In the end, it saves money and less headache.