I think main question is what kind of guidance are you looking for, and what is your budget.

If you are looking for only visual guidance, you can start on quite a low budget, and there is already a second hand market for this kind of equipment.

If you are looking for a guidance system that also controls your sprayer, there are solutions too... I think JD, RAVEN, AG LEADER, ZINX are some manufacturers to look at.

If you are looking for a steering solution, Trimble/AG Leader, JD, Raven, Autofarm are some manufacturers that come to mind.

The ultimate solution is a steering solution that also controls your sprayer: highest budget. I think all steering manufacturers have such a solution to provide.

Bear in mind that this is 'high technology' stuff.... So local dealer support may decide on success or failure of your plans. It's your money anyway...