Looking to build a laptop, though would consider buying a pre-built.

My requirements are:

1) $300 - $500 price tag
2) Linux OS (Debian or Ubuntu)
3) Decent Processing 
4) Light Weight/ low profile

This doesn't need to be super computer. I'm mainly looking for something to play around with programming and some statistical analysis. This would also be a chance to step into the realms of a full blown linux distribution. Trying to get off the microsoft/apple bandwagons as my experience with their products continue to get worse and worse every year. Additionally I have become a big fan of Linux though my experience is limited.

I will consider buying a pre built but do enjoy tinkering with RaspberryPis and would enjoy the project of assembly and modification.

I use a Dell for work and have been very happy, but I have no preference on brand.

A bonus here would be the ability to upgrade down the road. Ease of compatibility would also be a plus, but may not even be something worth heavy consideration.