For me, the X270 ist the sweet spot: it is small and portable and has a acceptable battery( in fact 2 of them). The display is usable and comes in full HD. 720p is a little bit too low for my tastes. You can upgrade the RAM and put an m.2 ssd inside(although only with 2x PCIe bandwidth). You can also charge it with USB-C. With an i5 it goes for around 200€ in Germany. US prices are usually lower.

Hubschraubereinsatz!