A pre-treatment machine will give you more consistent results. It will help you to get the best opacity from the white ink.
Uneven spraying makes for uneven results. There "is" some latitude between too much and too little, but you won't know if you're within that latitude until you actually print, or wash the shirt. By then it's too late.
Our industry is finding out just how extremely important it is for good controllable and consistent pretreatment laydown.
Is it worth it for you to spend $6,000 on a good PT machine? No one can tell you that, except for you. Add up your rejects, returns, reprints, wasted oversparay PT cost and labor for all of the above. Suddenly that $6k doesn't sound too bad.....
I can't say here which one is THE best, ask around our industry and you'll find out which one gives the best controllable laydown and is the highest recommended PT machine....
I don't want to start anything here.... but if you have "weeks" between jobs, you really aren't IN the DTG business. It must be a side job to round out something else you make a living on.... if that's the case, stick with hand pre-treating.