There isn't a single, cut-and-dried answer to your question. As you stated, unless you already have the appropriate inventory of tools, OEM terminals, connectors, loom and tape specific to the task and are willing to purchase spools of GXL/SXL wire in the correct colors and gauges (leaving you with unused excess), generally, you will be ahead by purchasing an accurately branched, prefabbed, OEM style harness. The biggest advantage in stringing your own wire occurs when you wish to deviate from the factory configuration (wire gauges, routing, soldering and introducing specially tailoring circuits that support non-standard accessories). Unfortunately, building your own harnesses can be tedious without a jig to accurately locate branches.
Still, many prefab aftermarket harnesses require a certain amount of customization due to production variances between vehicles. So, even with a prefab harness, you may encounter more than a straightforward R&R, wire-for-wire, connector-for-connector.
The wiring harnesses marketed by the site store are fabricated by American Autowire, a top-shelf manufacturer who adheres to OEM quality. Wiring harness availability is broken into sections. Email
Captkaos, the site owner, with a list of the harness sections you need, requesting prices, availability and delivery timeframe, before making up your mind.