What about a small - older second hand perhaps - notebook.
Which is exactly what I did. As I have a long history of using IBM/Lenovo ThinkPad laptops, I went for a Lenovo/IBM X60. In its day this was a powerful compact dual-core machine with a 12inch display. It's still capable of running anything you'd need out portable, and it is built like the proverbial masonry tŷ bach! With a newish oversized battery it lasts for several hours running a 3G dongle and KST2ME.
