Hi Balmerhon
I'd suggest at keeping the desktop machine for backup, spare purposes as well as being a 'central' resource. I'll explain a little more, By using the existing desktop PC as a 'main' PC you could keep all important documents, photo's, files, programs on it. Keep the printer (if you have one) connected to it as well.
When you get the laptop and the required Wireless Modem/router you can share those files, pics, docs & printer with the laptop wirelessly. It's not too difficult to setup and is an excellent way of expanding your computing power/usefulness without breaking the bank
As for the laptop - I'd agree with looking on ebay as well. Of course, be very careful, and only buy from high rated Power Sellers with a landline contact number. If you've a little more confidence, then buy from a private seller - again with a good rating preferably. Last week I purchased 2 ultraportable laptops from private sellers - no problems at all.
How much will you pay?! thats difficult - there's too many specs, sizes, features to be able to give an accurate picture here. I suggest you make yourself a list of must have requirements and a list of nice to haves and compare the listings. You'll soon build up a picture of how much once you see ending of auction prices , compare those to buy it now's from power sellers.
As an IT consultant, I'd recommend going for the best specified machine as you can afford. Nowadays it's possible to pick up a brand new Dell laptop for around £350 inc VAT and Delivery!
http://www1.euro.dell.com/content/products/category.aspx/notebooks?c=uk&l=en&s=dhsIt's a Celeron processor which is the budget version and not as powerful as the pentium centrino's and latest Dual and solo Core processors, but for light to medium use they're fine.
If you can spend that much, look for a power seller on ebay selling a higher processor laptop for arund the same price as a suggestion.
The wireless router/modem you can get from places like Ebuyer and Dabs.com for around £50 and some even have deals where for that kinda price they include the wireless reciever as well.
The only added 'costs' will be things like extra anti virus and anti spyware applications, as well as any others you may wish to have - some of these can of course be free (open office, Adaware, search & destroy, AVG anti virus etc)
Hope this helps !
Cheers! Dennis! West London & Slough UK!!