Hi
Even the smallest 'residential' system can be used for this type of Job. A complete 12ft kit (which will get you to 20' high) includes the pole, brush, hoses and single resin cartridge and simply plugs onto an outside tap, costs £99 plus vat. Depending on the size of conservatory, how dirty it is and how hard the water is in your area, you can expect to clean 2 - 3 conservatories before you need to replace the cartridge (£24.50 each). Providing you can work this cost into the job then this is all you need to get started!
As you do more work you can upgrade the system and continue to use the 12' pole but with a system that can produce more water at a lower cost.