Thanks!

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).

Don't hesitate to ask more questions ;)