Not a fan of nylon. It is not consistent in size, stretches, shrinks, gets stiff, rubs some horses raw, fades, gets rough, then breaks. By the time you buy a harness, then replace the pieces that don't fit or did not come with the harness, you could have saved up and bought a good complete harness.
A decent Amish biothane buggy harness can be had around here for less than $400. I bought one, and use it all the time with no wear and tear. I imagine I could get a used one for $250.
You also need to figure out what kind of carriage you will have, because that will impact what type of harness you need. The trace connections and tugs come in various styles, depending on your carriage.