Fair enough a 3.5" HDD will pull another 10 watts of power but put it this way: Look a the power rating of a single lightbulb.... think the generator can take it? :P You're going to be kicking yourself if you get an 80 gig HDD and the slim DVD will cost you £100 more on top of the price of a similar spec full-sized drive.
If you want a small, power efficient system just get a laptop witha USB 2.0 TV card :)