I don't know I can offer any good advice because it's going to depend a lot on your circumstance and preferences. Decide what you want and run the numbers yourself.  I'll use Northern Tools site just because it's easy.  A 55 gallon spot sprayer with a 2.2 gallon 12 volt pump ready to go is $400.  


The 55 gallon skid with Honda engine with a 5 gallon 350 psi pump and 150 foot hose reel is better.  But is it worth $1750 to you?
 
You can buy a 50 gallon tank for $220 then add a 12 volt 5.5 gallon pump for $65 and another $50-$100 plumbing it.