I would say definately buy a refurb but make sure its from a reputable company. My brother picked up a new laptop the other day and the guys in front of him we're buying refurbed Acers. Well by the time my brother was served the three guys were back in for a refund as there were panels hanging off the refurbs.
Within you're price range you're probably looking at either : Toshiba NB100-12A, Intel Atom N270 1.6Ghz, 120GB HD, 8.9" TFT , XP Home
Acer Aspire 1 A150 blue Atom N270 1.6Ghz 512MB 120GB 8.9" screen camera XP Home (which is still £250 - but you might be able to find it cheaper elsewhere) (check if you can put more RAM in, as 512mb is well behind the times!)
Whatever you do avoid these:Z-Arianet 7" Minibook with Windows CE
Elonex VOIP Netbook LNX-CEWS72 VIA 1.2GHz 1GB 30GB 7" screen
The Vodafone deal you are looking at I would personally go with the Samsung over the Dell, but thats as I feel Dell's need customising on their site to you're specs to make them worthwhile. And that ends up extremely expensive for what you get. And as I said above the Samsungs can usually take some hammer, so good if you're on the road. Also worth mentioning on Overclockers the Samsung is £300 and only netbook on that site with 5 star ratings.
Also I think its £30 per month for the Vodafone deals on 3Gb connection, £25 for 1Gb (on their website at least), and thats on a 24 month contract.....which works out at £720 in total!
Check the following sites for refurbs or otherdeals:
Microdirect
Aria
Overclockers
Summing things up and without putting you're hand too deep into you're pocket I would do some research into free Wi-Fi connectivity (places like Starchucks and McShitdonalds, and service stations often have free WiFi access), and pickup a netbook yourself. Also I think T-Mobile run some sort of Wi-Fi hotspot service, not sure on the coverage tho. Do you mind me asking where you will be on the road?
Not sure on the whole power invertors front however I do know my brothers friend was running a laptop in his wagon with no problems, using some type of invertor. Best advice, get something thats more expensive or with great reviews so you don't fry you're netbook.
Let me know what you think and if you need further info

Peace

E2A: LG GE20NU10 20x DVDRW/RAM external Drive is £35 on Aria, should sort any lack of disc drives out for ya!