I can say that the GTX 1070 is strong enough for handling 144 fps on 1440p resolution. I am not sure about the cpu, as it is a bit old and can be bottleneck the gpu. But if you upgrade your cpu, then I don't see much of a problem. Competitive games are good to go at 144fps, but you cant at highest settings. For actual single player games, 144fps is a bit too high. So you have to lower the settings to "say" medium. But even if you get only 100fps, it is better than 60fps.
And if you really have problems with certain games, then you can lower the resolution on that game only. For test if your machine can handle the bigger resolution, try the DSR from Nvidia Settings. It simulates your machine higher resolution, your pc works as if it would have that higher resolution, but downscales it for displaying on lower resolution. This way you can see if your machine can handle it before you upgrade to a monitor with that resolution.
About G-Sync, some say its essential, some say its not. If you ever had tearing in games and you set V-Sync on, then you know how useful G-Sync can be. Because V-Sync on will introduce some input lag, which is important on fast paced online shooter, where you better V-Sync off. And here comes G-Sync, which will have low input lag like V-Sync off, but will have synced images like V-Sync on for eliminating tearing. Its up to you, it can't harm. Also, if you have V-Sync on say at 60hz/fps and your machine cant handle it, then your machine slows down. With G-Sync it can drop fps without having any slow down effect. Hopefully this wasn't confusing.
To me personally G-Sync is not essential, but a useful addition if you can get it. Now its up to you, if you want bigger monitor or G-Sync. I think, I would go with bigger display. But I am happy to have G-Sync too. Why don't you look at 1080p/144hz/27"/with gsync?