You dont say if it is new or second hand. A Windows Vista laptop needs more power than a Windows XP laptop.
There is no such thing as a miniumum spec, laptops just get slower as the spec goes down, but they are still usable.
For the interent and iplayer the broadband speed is probably more important than the laptop spec.
And word and excel on their own dont use a lot of memory.
If you can tell us if you want to buy new or second hand, and also an idea how much money you wish to spend, we will try and help more.
Also, I would not buy a second hand laptop, it is too risky and not easy to spot if something is wrong with it (may have had liquid spilled on it which has rotted the inside for example)