Dandan this might help,
What you are looking for is reliablity.
The Pressure Washer
If you go with a petrol engine then Honda has to be the best really.
13hp machine is a work-horse and will run all day for you. Also petrol engines are usually cheaper and lighter then diesel.
Pump
Interpump or CAT both tried and tested. You want about 15 litres of water per minute, anything less can be a struggle. PSI around 3000 or better, you can always turn pressure down, not up.
Flat Surface Cleaner
Choice of three really, Whril-away, Tech-clean, Mosmatic.
Lances/wands
If you have a US style lance with various tips/nozzels you pretty much have anythnig covered, just add a turbo nozzle.
I prefer, a dual lance and a seperate turbo lance myself.
High Pressure Hose
Most set-ups come with a 10 metre hose as starters, but well worth adding a 20 metre hose, with Q/C connetors to have a total of 30 metres.
You will also need a holding tank of some sort to supply the washer with water. Most pro set-ups draw/suck water.
Hoses to fill and supply tank/washer from water supply.
That's pretty much the basics.
Once you have started you will always find something to add/aid your work load, but the above is pretty much what you need to get started.
Good luck......go for it.