The core of precision farming is informational, not mechanical: knowing what happens in the field and not treating different places the same. A sensible path for a 100–1000 decare farm: (0) digitise the records you already keep; (1) satellite monitoring — Sentinel-2 images every field every 2–5 days for free; (2) soil zones from satellite analysis at ~0.19 €/dka with 2–3 lab samples for calibration; (3) variable rates with the terminal you already own (much post-2015 machinery reads prescription maps); (4) only then think about new hardware.
Under 100 € for the first precision-farming season, without a single new bolt. Start here.