I think the fact that the shell is starting to give is a good indication to buy a new model. By the sounds of it you would be fine with a Macbook Air, although I would recommend a 256GB model which is quite more expensive but at least you won't have capacity problems any time soon.

I have a 17" MBP from 2007, and am seriously considering replacing it now the latest models have come out. The jumping point for me has been a substantial change in hardware. I.e. only now can you get a Quad-core laptop from Apple. Since it's their first quad-core I won't jump just yet, will wait for 6-12 months for the first few batches of overheated quadbook pros to melt and designers forced to fix the issue.