Keep in mind that the default zoom level on your SP is 150%. This will extend to your monitor until MS gets around to fixing that. This means that things will be BIG on your external monitor. On a 27" monitor they will be REALLY big. I went with a 21.5" HP IPS display from BestBuy that was on sale for $129 and am quite happy with it. You can find them even cheaper on
Amazon. One thing to keep in mind when monitor shopping is do you want reflective of non-reflective surface. Non-reflective is good if you have lights behind you as they won't reflect off the screen. Reflective will give you a brighter crisper picture.
Couple options to
make things less big on your external monitor:
1) Set your zoom at 125%. Makes things a bit small on the SP, but better on the external monitor.
2) Change the resolution of your SP to 1600 x 900 and set the zoom to 125%. You probably won't notice any degradation in SP image quality and 1600 x 900 x 125 gives you same screen real estate at 1920 x 1080 x 150.