Subject to the usual caveats about buying second hand electronics:
Netbook seems good value, perhaps even too good a value. How old, and have you seen it? At the end of the day, it's whether you trust the seller not to con you with a "pup". Bear in mind it is a NETBOOK so whilst it may do the basics (browsing, word processing) OK, don't expect it to do anything very clever, or very fast. On the plus side Dell tech support are pretty helpful, although presumably it's out of warranty, so you can't expect more than very basic help, for free.
The GTX480 is a good card, up there with some of the best. £100 is a fair price, assuming it's in good nick. Again, you're back to how honest, and reliable the seller is. Again you are fortunate in as much as EVGA have good reputation for technical support, should you have any issues. Also, some of their kit has "lifetime" warranty, but you need full model number to confirm that.
Good luck with your build. I'm in the same process myself.