I would recommend Petrol or Diesel as it is less restrictive in respect of jobs you can take on.
Honda with Interpump are great. You'll need a minimum of 11bhp, 200 bar (3000 psi) and 15 LPM flow.
Go for a low speed pump driven via reduction gearbox as this will not be running at full pelt all the time and thus last longer.
For more details I'd suggest that you do a search as this subject has been aired many times.
If you want solid advice before you commit to buy speak to Malcolm Smith who knows a thing or two! He will help you with a complete package and then some.
Malcolm has a website: -
http://www.malcleanse.co.uk/Tell him I told you to contact him and said that you'd be OK for a bit of a discount :-)
Best of luck
Ian