for spraying, why not add a foam marker, when broadcast spreading make a pass and have someone walk alongside the spreader to determine broadcast distance, moving in or out as needed to find the edge of the spreading distance. Pace off that distance from you're wheel tracks and make your next pass that distance from the existing broadcast pattern. Sight on a far away tree, fence post etc to hold a straight "furrow" so to speak.
The last farming tractor we purchased, gps was expensive, 10K plus. now granted that was a complete package, steering, alarms the whole ball of wax, but i'd suspect even a simple gps thats positioning only will be more than most people want to spend.