hi
i wouldnt use a karcher, get a honda driven 9,11 ,13 hp engine driven as you have been offered. you really need 3 or 4000 psi from it
you will also want a turbo nozzle on your lance. some also use a rotary attachment like a whirlaway. not essential but on nice flat work like slabs it is quicker i find.
you can use just water pressure without chemical, some pre spray with a good alkali cleaner then rince away on low pressure setting.
good game to get into i find it better than window cleaning. get some decent breathable water proofs otherewise you will be soaked to the skin.
Rob mcclean posts on here i find he is the expert on this game and was a big help when i set up. see if you can pm him iam sure he will give you a few pointers.
cheers
john