According to my sources (and I have a detailed answer on SU on the subject, including a handy graph and links to a spreadsheet) - 27" is *pretty" darned close to the sweetspot for PC use.
I game on a 980TI at that resolution (which is fine) but I've driven 4k60 off much older/less high end video cards. I don't use gsync (which is mainly for gaming) and vsync is unrelated. Personally I don't think g-sync would be that useful here.
Personally I'd suggest the dell P2715Q. I have one and its glorious. Its the "sweet spot" 27" which is just right for the distance I sit at - roughly 1.25m. Its IPS (and this matters in terms of colour quality), and comes calibrated out of the box.
The wirecutter considers it the best 4k monitor of any size - and its reasonably priced compared to most options.
It has the essentials - pair of DP 1.2 ports (one micro and one standard) and one more to let you daisy chain (Don't daisy chain) , HDMI/MHL (4k30 only tho) in case you need to plug in a non PC device.
It also has a few nice things like an onboard USB 3.0 hub and the ability to turn off the powerlight when its in use. This sounds like a minor thing but its awesome.
It also comes with the dell stands - which are pretty much the best in the industry and let tilt, pivot and adjust heights to your preferences. You can trivially swap these out for standard VESA mounts for wall or multi monitor mounts, or a dell 2 monitor stand.
Fonts are readable (at 150% UI scaling on windows 10) and seriously crisp. What 4K content I've tested so far looks great too. I've basically ploughed through several novels and a few hundred stackexchange answers on this and its not fatiguing at all. I've gamed on it (though my 980TI sometimes wheezes during particularly demanding scenes)