Honestly, if you want to buy a desktop buy a desktop, a laptop won't be a good substitute.
This leaves you at following choices:
- buy a current iMac specced out to the point that will last you for several years
- buy a (current) base model iMac to keep you running till the next update and than upgrade (swalloing up some small loss)
- buy a used/refurbed iMac at better than base model spec, might even last you for more than till the next update (and even if not replacing it with a 2012 iMac won't generate that much of a loss)
- buy a current well-specced iMac and swallow up some big losses when the 2012 hit the store
- buy an expensive (and slower) laptop + monitor which will generate even bigger losses if you go for an iMac later on.
But ....
For the apps you listed (lightweight web use) you could easily do as I did last year (shortly after then 2011 iMacs came out).
Buy the base 2010 21" iMac and be happy for years

Yep it's 2 year old tech, but sofar I had no performance issues for normal desktop use. I would advice this route even more as you will be new to OSX and while most users "stick" with it some might still not find it to their licking.