I think it is better to buy a good rim rather than disassemble an existing front wheel and rebuild it as a rear. You need new spokes and new 36H rear hub anyway. There are many good rims, like this one:
https://www.ryde.nl/andra-35 or Andra 321 for disc brakes, Velocity Atlas
https://www.velocityusa.com/product/rims/atlas-622 and many others.
When I built my rear wheel using Sapim Force/D-Light spokes and Shimano M525 hub with Alex DH19 rim (no eyelets, the cheapest version) I regretted it right away. The rim was <$20, and it was the cheapest part. It works, and there is nothing wrong with it, but i could have gone fancier.