I guess it would depend on what you are spraying, ground, trees or both. If you look thru the Agri-Supply catalog(
www.agri-supply.com), you will find quite a few options. And like Berniep stated, you will be hard pressed to beat some of the prices if you build up your own, especially if you consider your time involved in "re-inventing the wheel". I bought their plain 25 gallon spot sprayer tank/pump combo($138). Once you figure in all the piece parts and the shipping + time to assemble, it was hard to beat. This one already has the pressure gauge and a "Y" gate valve installed so adding a boom is about a 5 minute job(one side recircs to the tank for controlling boom spray pressure and to help with mixing and pump flushing, the other connects to a boom). It also has a 30' hose and wand plummed in near the gauge so you close both "y" valves and the pump cycles to 60PSI and stops automatically for spot spraying(pressure switch maintains 40-60 PSI automatically).
They have a basic 2 nozzel boom with 80" coverage for about $48(#56427). It comes with all the hardware/brackets and could be easilly connected to the bottom of the tank and the whole affair set in a FEL bucket. You might have to extend some wiring to be able to control the pump from the seat. That pump size will suport 3 of those type nozzels found on the 80" boom. That boom is also made by Fimco and I got the part numbers off the FIMCO website(they have all their manuals and parts lists online) and ordered an additional nozzel, screen and screw cap from them for that boom. Agri-Supply has the 1/2" hose to nozzel fittings that the nozzel screws into as well as the boom clamps to connect it to a 1" pipeboom(all inexpensive). I ordered up a "T" fitting and clamp from them for the center nozzel and converted the rest of my parts from the 2 nozzel 80" coverage boom to a 3 nozzel 120" coverage boom. That spray tank with 2.1GPM pump will run the three nozzels at a near ideal 20PSI/.47GPM without recircing any fluid to the tank. When at the proper height(18" above ground)Those Nozzels at 20PSI and moving at a comfortable 3MPH deliver 20 gallons per acre which is a good mix rate for the 2-4D that I lay down(1QT/20GAL/Acre) for weed control. When spot spraying with the wand, 20 gallons goes a long way. As long as your trees are not too high, that wand should work for light tree spraying also.
If I had it to do again, I would build a 3 nozzel boom from scratch to start with using those parts from Agri-Supply(one "T" and two 90 degree 1/2" nozzel elbows and brackets) and Fimco(three nozzels, screen filters and screw on retainer caps from the 80" coverage boom) and provide my own boom pipe(which I did anyway) and hose to connect it all together for about the same price in parts as the 2 nozzel boom that agri-supply sells.
Good luck