Typing on a laptop on your lap on a bus is always going to be difficult no matter what you buy. But maybe you don’t need a laptop:
I find it really helps if I edit my work in a completely different format to the one I wrote it in. If the work looks completely different I’m less likely to be blinded by the familiarity of my own words.
I read somewhere that when you look at your MS on a computer in a neat font, it gives the impression that the work is perfect and it’s harder to see problems when it already looks polished.
So, what I do is change the font to something really gritty, like American Typewriter and print it out.
I use editing marks for changes (these are easy to learn from any good editing book like the Wiley Style Manual) and then transfer them from paper to laptop later.
It may seem like a waste of time to use paper and have to transfer the edits, but what you end up with is actually a third draft, because you naturally edit/reword again as you implement those changes.
If you don’t have a printer or feel it’s too much to print out, you could invest in a tablet and a stylus. I edit like this sometimes too with an iPad and a Jot Touch. But you still need to transfer the edits to the computer later.
It will be difficult on a bus, but I think reading your work aloud is essential. It is only through reading aloud that you can isolate difficult to read sentences/paragraphs and really get a feel for your voice and the musicality of your words. Maybe you could sit at the back where it’s quiet and whisper it to yourself!!