As Poleman says, off the shelf doesn't have to be expensive, you can be up and running for a TOTAL outlay of no more than £1,200 to £1,500.
www.freedomwfp.co.ukhttp://www.window-tools.com/There is a link in one of the other posts for Gardiners, these sites will give you some idea of the outlay needed.
The big boys at Ionics and Omnipole and so on all do really top systems, but they are not cheap.
It really depends on the depth of your pocket.
If you are into DIY then go that route, for a few hundred quid you can be set up and ready to go, but it does take effort.
Matt's DIY site is a very useful guide, taking you by the hand and leading you through this route.
http://d.co.uk/If you CAN afford it, then go for a ready made system, what you eventually go for will then depend on where you live (Flat, house, do you have a garden, how for you are from your water supply and so on) and the type of work you have.
You have the choice of backpack, trolley system, trailer system of van mount...or a combination of all three!!
Take your time, there is a bewildering set of choices to be made
Ian