That answer will be based on personal preference. I would say that most of us, that being about 80% or more use downstream injectors for most of our washing. Some use the xjet and some use dedicated sprayers like 12 volt or air compressor diaphragm pumps or 115 volt booster pumps.
With the system you will be getting I would call Russ at Southside Equipment and order a couple of the General Hi Draw injectors with rebuild kits for acid service (ceramic ball, o-ring and spring that will hold up better when used with bleach) or order the acid injectors that would already have the parts in them but get the rebuild kits so you have them and not shut down and have to order when the parts wear out.
I would order the Jrod nozzle holder for your nozzles so you have 4 nozzles there in one place. I would say probably a 40 or 50 orifice for downstreaming but the spray fan patterns is personal preference. I like to use the 1540 for spraying close with a tiny bit of pressure to move dirt and 0050 for spraying up high but some guys like the 2540 or 4040 as the fan is wider and you can cover more area faster.
The nozzles are not much so get some and see what you like. I also like to rinse with the same nozzles so I have to go take the hose out of the container of SH so a lot of walking back and forth to the trailer on the job. I put the hose into a bucket of water so it is rinsing out the injector while I am rinsing the house but some people like to use high pressure nozzles to rinse, all personal preference.
You might order some of various size nozzles and a couple of the jrod holders so you can have different assortments of nozzles based on what you like and what you clean.
Hope this helps.