Aight.
For wireless internet, unless you are willing to spend a bit more I reackon, you'll need to buy a USB adapter, or a wireless card if the laptop takes expansion cards (A big slotty bit on the side).
Windows 98 is the minimum really, though it'll need drivers with USB stuff, not really much plug and play support. XP would be much better. I'd want to spend about £120/$125 or so. I think that's roughly the exchange rate now.
But, £50 is do-able. Old Dell's are pretty sturdy and last FOREVER it seems. Batteries not so much. Grab a cheap one with win98 and get a $5 USB wireless adapter, it'll have a driver CD, install that and go!
Also go into town and leach company internet for free, win-win!