I've had a 60gallon cube running since Feb 07. It's a Visio, and has seen a bit of an evolution over the years. I started with a a pretty unique overflow design that spaned the entire back wall in a shallow V with just enough room between the back pane and the apex of the V to fit a durso. To keep the profile of the overflow shallow, I plumbed the durso into the back wall at the base.
Fast forward 2 years and I pulled that overflow out, capped the bulkhead and went with a glass holes 700gph overflow. I had some noise issues, but we've got that figured out now, and I'd say for a cost effective cube that looks nice and has as much space as possible, it's tough to beat a cheapo Visio and the glass holes overflow. Corner overflows are nice, but they will take up a lot of valuable footprint in a 24x24. I'd really look into an external or glass holes overflow to save space. On top of that, the beauty of cubes is the 3 or 4 sidded viewing offered. Why clutter up a viewing pane with an overflow box?
The back of my stand is open, so i've been able to run a 20tall tank for a sump. I don't think you'll want to mess with the 19X19 cube as a sump because it will be difficult to baffle a return chamber and still have room for some of the better skimmers out there. They may not be in the budget now, but if you stay with the hobby, you'll end up going that route, and it's better to buy once/cry once whenever possible. Try and plan for the evolution of your needs/budget so you don't waste money on avoidable incremental upgrades.
Depending on what you want to keep, flow can be tricky in a 24x24. I've got it sorted out now, but it took an MP40 to get there. If you want to go all SPS, or all LPS/softies it can be done with less expensive alternatives, but to keep both in a small footprint is a balancing act.
As far as the tank itself, I'd explore your options. I have a "stock" 60g Visio cube. IMHO, the quality isn't that great, but I also didn't pay much for the tank. The edges are a little rough, and all in all, I've seen much nicer work, though it obviously cost more than my Visio. If you're going to go rimless, make sure you factor in 1.5-2 inches of pane height that won't get used. Still, I think 24x24x20 rimless would look better than a 24x24x24 rimmed, but that's just me.
It looks like your sold on going sump over AIO, and that's good. The extra water volume and ability to have an area for maintenance etc. is important. I know the budget may not be there right now, but that Cadlights 50 IS nice, and it's a comprehensive setup. You can DIY for less, but in the process you'll likely get upgradeitis and spend more over the long haul.
I guess the point I'm trying to make is, get what you really want the first time. Even if it takes longer before you have a glass box that you throw money into, you'll be happier in the long run. I started this hobby with a 40gal acrylic display and dual 175 hallides I got from a friend. There have been few equipment purchases to this day that cost less than my first tank and lights

If you decide to build your own, be very complete when planning your budget. Things like plumbing, electrical, and materials for your stand add up quick. I spent 2x the cost of my current cube on the wood to build my stand and canopy (no longer run the canopy). If I were to total all of my hardware purchases for my cube as it sits today, I could buy the cadlights 50 and have a lot of money left over to start stocking it.
Chris