If doing walls (smallish) area we use backpack - Gardiners or any off fleabay - they are as little as £65 - so far they have lasted over 18 months with no issues.. large areas them mixer of pressure washer
for drives and patio's you can't beat IK9 sprayer - made for the job - the trigger will get rusted quickly if not cleaned but if you search the makers name ( sorry, currently got a senior moment going ) they do the same sprayer in blue for car valeting which has an all plastic trigger - the parts are available and cheap for these sprayers as well
Darran