mark,
I wasnt getting at that i was just saying it seems a bit of a waste of money to buy one just to use it once a year.
If you do want one though just for this purpose though then a cheap one (disposable) should do the job.Presuming you wouldnt be buying a rotary washer and just using a lance then i would think that any machine would do, just make sure it has changeable nozzles and not one with just a pencil jet on it.It would take weeks using one of them.
Dont really know what pressure they put out though.
If you think you may use it for business aswell then get the best you can afford,there have been loads of posts about which washers are suitable for commercial use on here already.
good luck.