I built my 3 point mounted 2 row sweetcorn planter in about 1 afternoon. I use a 12 row for my field corn, but wanted a small one for sweet corn. Found an IH planter for parts and bought the units (IIRC, they're Model 295 units) for $75 each. I mounted them to a piece of Deere tool bar I had, had the 3 point brackets from an 8 row cultivator I scrapped out. Put it all together and it's done. If you build one, make sure you get the row units that drive from the closing wheels. Most planters have a separate drive wheel that drives several row units.
As someone above posted, you might be further ahead to buy an older 4 row. My brother-in-law bought an older Deere 495 4 row for $300. Might be more plentiful in your area.