Chromebooks are a lot different compared to a laptop running Windows. Chromebooks basically has an internet browser as an operating system. There is no option to install programs like a Windows laptop and printing from Chromebooks is not as straight forward as a laptop. Chromebooks has it's uses, but I don't think it is appropriate for web development at all.
The T420 is a good laptop for what it is, though it is rather dated since it is 7 years old. I recommend buying the following Acer laptop for $380 which is more powerful than the T20 and being new, it has a full warranty.