if you are going to use it on a regular basis then its well worth getting but if your only going to use it very occassionally then id suggest getting it done by a good valeter. if your wanting to get one then look for the numatic goarge machine as its the best tool for the job, ive had mine for a long time now and its shampood alot of customers cars and is still going strong now. your looking around the £180/200 mk, you'll want the machine that sprays and extracts, theres 2 varients, 1 is a wet and dry only which doesnt have the pump for spraying the shampoo and the other has the pump which will be more expensive so check before you buy.
failing that if your willing to travel down to me(im near gatwick) i'll do it for you.