I've had amazing luck with my Achevia QH270 27" 1440p display. I purchased it almost exactly a year and a half ago for $289.99 with free two day shipping from Korea. I upgraded from a 3 year old or so 24" Auria (an awesome monitor for the time and price) and my 1.5 year or so old 21" Acer monitor (it was ok, I preferred the colors of the Auria though). Both were 1080p displays and I was quiet hesitant to return to a single monitor after using 2 for a year. After my roommate purchased a QH270 it took me about 5 minutes of looking at it to decide to sell my monitors and order myself one.
I felt pretty much no loss of screen real estate, in fact it feels like I made gains. My computer setups have always been small desks so having two monitors I had a pretty healthy tilt on the secondary display which cut down on it's usefulness. Snapping two windows side by side on a higher pixel display is much better for me then having two different screens.
I definitely see the allure of multi-monitor gaming with an eye-finity type set up, but the bezels just kill it for me. I know their are some cool mods and monitors with barely there bezels, but I still don't like it. Until they make true bezeless/curved displays I'm going to be sticking to single high pixel monitors, unless I have a need for a dedicated second display.
I've looked into 4k a little bit, my next monitor is most likely going to be a 4k display. I'm hopping my current display will last until I have the money for a new build and the monitor itself. From what I've read I'm not sure if a single 780 would have enough power to drive 4k gaming. 2 on the other you'd probably be good to go. It's all relative though, I'm sure people would say that my 2GB 660 TI isn't enough for 1440p gaming, but I run nearly everything on max and have good FPS. Thus being said I guess the most GPU intensive game I've played would be Battlefield 3.
Between 3 1080p monitors and one 1440p monitor I'd take the single 1440p every time. You'd have less pixels overall, but everything is just so much easier, one monitor one set of cables, much easier to move, etc. If you go the 1440p route a single 780 would be sufficient for basically everything. Then you could always throw in another one down the line if you want to make the jump to 4k.
I used to be a huge multi monitor fan when I was running 2, I was even debating getting a third, but after going to the single 1440p I doubt I will go back. If you have a chance you should try to check out higher resolution displays, though it may be difficult to find a good place to do this. BestBuy's usually have some iMacs, you'd be able to get a feel for the resolution at least.
As for MST isn't that just a feature of display port 1.2 for using multiple monitors on the same cord in a daisy chain or something? I don't think it would be an issue since he would most likely only be running a single 4k display.