I recently had the urge to get a fun, colorful summer watch. But I can't justify the expense of even a beautiful, beautiful Tsuyosa right now, so I built my own for less than 70€ :D
Built around a Seiko/SII NH35, all parts (case, bracelet, dial, hands) sourced from Aliexpress. Fit and finish are OK (I'd say a bit below pre-reboot Seiko 5), but these are very cheap parts anyway.
It's not that hard really. I made about half a dozen Seiko 5 mods and small repairs with basic tools.
I'd say:
Get a cheap watchmakers toolkit, it's more than enough for this purpose, plus some good wire cutters, Rodico watchmakers putty and lint free cotton pads for nail polish.
get familiar with the terminology so you know what to look for and which ones you need (eg. movement, stem, crown, dial, hands, chapter ring, case, crystal, bezel, bezel insert etc.)
because the Seiko 5 modding scene is huge, start with a cheap standard Seiko movement like an NH35 or NH36, and search for compatible case, dial and hands. Most aftermarket dials are the Seiko 5 standard 28.5mm, so most hands and cases assume this diameter.
A good starting point would be an SKX007 base as it was the king of modding. Unfortunately, like all "real" Seiko 5s, it's not made anymore, but there are a truckload of compatible parts in a infinity of styles still being made, including cases.
you can get cheap but random quality parts on AliExpress, or very high quality but rather expensive parts at reputable online shops like Dagaz Watch, DLW Watches, Namoki Mods or Lucius Atelier.
The hardest part by far is fitting the hands, especially the seconds hand, the rest is trivial in comparison. Even cutting the stem to length.
There are video tutorials everywhere on YouTube to help you on the nontrivial bits (inserting and removing the stem, fitting the crown, fitting the hands etc).
Oh wow, I much fuller reply than I expected. Thank you! I did read up on some stuff already and saw the nh35 suggested so I started looking for parts with that listed in the title. There's no gotchas if I stick with those? Or do i need to make sure the parts fit together?
Just make sure the case either fits a 28.5mm dial or takes a standard chapter ring. That's probably the only caveat. Parts from the reputable shops are guaranteed to fit so it might be more reassuring to start there. Parts on AliExpress need careful examination of the listing and customer pictures.