You’re going to be struggling a bit with wheelbuilds so narrow without going and seeing someone or learning to do it yourself and going crazy in CRC’s sale. Building really isn’t hard if you’re methodical and organised. There isn’t any essentially equipment other than a spoke key. Everything else can be done with an upturned bike frame.
OTOH, if ‘fitness bike’ means the same to you as it does to me, maybe just get something off the peg like a set of Crossrides? They’re still only 21mm with the ‘width upgrade!!!’, the hubs aren’t loud and they’re pretty cheap at around £150 a set and upward depending where you shop…