I did my own WH-00125
I bought all of the stock materials from electronics suppliers and constructed a wiring layout board to build my WH-00125.
All in all it turned out to be more time and effort than I thought it would be. Took me about two months to do the actual construction after I figured everything out (not full time) and I probably saved a bit - not sure of the actual savings. Now that Stein has had a few more years to address their design of the kits, I would give one of their options serious consideration - IF they are still offered.
When I did it about two - three years ago Stein did offer three options; 1) a fully complete harness, 2) a kit with wire cut to length, connectors, and I guess - instructions, and 3) a package of the raw materials. When completed, installed, and I began burn testing everything that I connected - it worked, and I only had 1 more thing I had to wire in, that was a power wire to the Garmin heated pitot.
What I can offer now is to allow about 6" more length to all of the wires that extend up into the panel area. And, study the wiring harness diagram and identify any additional wire runs for future items that might get installed - it is a big job to run those wires after everything is buttoned up in the cockpit fuselage belly. If you go with a Stein option try to confirm with them that adequate lenght is a feature of what they offer.