I wouldn't try Ubuntu on the Surface 3 Pro. I'm pretty sure you will have problems with the keyboard ... like these guys here:
http://www.geek.com/microsoft/linux-users-rejoice-heres-ubun...
Don't get me wrong, I like the specs of Surface 3 Pro, I just don't think it will work so great with Ubuntu.
What you should do is to do some research about how Ubuntu runs on the hardware other people suggests.
Personally I would go with a Dell XPS 13" that already comes with Ubuntu 12.04 installed. The system will probably work well with Ubuntu 14.04 too. Also it has 8GB or RAM and 256 GB SSD.