I use a mid-upper range one. It's good enough for teaching Android, but not enough for full time work.<p>SSD and a fast modem are a must. Second, I'd recommend at least 8GB RAM. It'll give you 5-10 min build times, which is a pain for debugging, but at least you can build layouts and write code without it freezing.<p>HP Pavilion is my pick. Keyboard and touchpad is a little janky. But the backlight helps a lot at night and it's gives the best power per dollar.
A lot of makers have a corporate line and they tend to be good choices because they're robustly built, the documentation tends to be better, and they have better availability of things like chargers or batteries. You can usually add ram and an SSD to turn a modest machine into something very usable.<p>I like the Dell Latitude range.<p>Toshiba have either Tecra or Portage.<p>Lenovo have Thinkpads.<p>HP have Elite and Probook.
I would recommend a second hand business Dell or Thinkpad with at least 16GB and a SSD.<p>If you would rather buy new, think about USB ports --especially if you have an external keyboard.<p>I have had less problems developing on Linux than other devs using Macs but if you also want to develop for IOS then a Mac is the only choice.