Except when less heat means doing without the fan control you need to keep noise down.
So it might be good for noise to pick an Asus board even though they run hot.
and that the computer will be up to most tasks - excluding gaming - for many years to come. In other words that it will have some 'computing reserve' besides ordinary web/media stuff.
The best way to achieve that is to spend less on the parts today and save the money. You'll soon be able to buy used parts to upgrade your computer for pennies on the dollar...
Buying a 1155 board that's compatible as many 1155 CPUs as possible means you'll have an easier time upgrading the CPU.
But yes, I do at the same time want a computer that is using less power than the average computer.
Your choice of 1155 CPU won't affect that. Today's average computer would use more power than yours even if you picked a quad-core down the road.
In order to keep heat and noise down, underclocking is sufficient unless you're planning to constanty crunch numbers with 100% of your CPU (it sounds like you're not). Underclocking in software is easy and reversible anytime in case you want to unlock your CPU's "computing reserve" for a particular task.
By the way, underclocking a quad-core gives you very good multi-threaded performance relative to the heat generated.
That's why I recommend not to waste your time and money on undervolting or the *T or *S CPUs.