Do not buy the xjet nozzle. It's nothing but a pain. It actually contradicts itself in the way it's designed... that's neither here nor there...
"PSI" is the power rate. the "GPM" is the flow. Your 2000 psi mickey mouse machine will work fine right now. Your aim is going to be a 13 hp honda. Then you can choose your pump (3500@4.5, 3000@5, 4000@4, etc). Your spray tips are going to be 65 degrees (for tile) and 80 degrees (for shingle). If you're using any other tips on those roofs, get off the roof immediately. Go to the truck, and resign. Roofs aren't meant to be blasted. Your chem choice should be non-chlorine "brand" roof cleaner. You don't want the sodium hypochlorite, you don't want the sodium hydroxide. Get the "Roof Cleaner" stuff (whether it's "renew", "shingle shield", "roof & deck", "roof-brite", etc etc). All of them need 1 hour to sit. My favorite besides roof-brite (exclusive to us, nanner nanner poo-poo) is the home depot roof cleaner. Works great, is awesome for low pressure cleaning. Shurflo is a pain, just like the xjet (by the way, the xjet is just a rip off... it has NO business on your gun), but if you want to control your mixture, it's the way to go. If you're ok with pulling 20%, get the 5:1 injector. Either way, one hour set time!
You'll buy a new gun every two months.
Buy a high pressure ball valve & attach it between the hose and your gun. This allows you to now switch guns & surface units w/o shutting down the pump and turning off the water. A gun w/o a trigger assbly will last forever with the high pressure ball valve as your trigger.
Housewash, roof maintenance (fungicide app), small patio, pool/cool deck:
Here's the tool that saves your arm. I created mine with a high pressure adapter. The adapter will get you from a 3/8 qc to a 1/4 qc. Traditionally, the gun is the adapter... the hose comes in and fits the gun with a 3/8 qc, and your spray tip @ the other end of the gun fits into a 1/4 qc. You place the adapter between the two, and you have a 4 1/2 inch piece that fits in your pocket. Makes for the easiest chem apps. When doing homes or soffits, or anything else, you can attach a 2 or 3 foot alum lance to the piece, and then pop the spray tip into the end of that. Your valve is now the trigger, but be careful... If you let go, all H*E*double hockey stix will break loose!