I have demon broadband, its 512, unlimited downloads, and is ultra reliable. I have had it for about 6 months now, its probably been down once.
Personally i wouldnt get one with a limit, i nearly did, decided against it, now im glad i didnt.
I will sometimes download 2gb a day let alone a month if there are a couple of films i want to watch!
You will be amazed once you get it how much you will use it.