Tip: 7S26A is the official name for this Seiko calibre. However, most sellers refer to it as just 7S26. It is the same thing. Equally suitable for this project would be 7S26B or 7S36.

Which one to buy?

With so many watches featuring a 7S26 movement, it would be tricky to pick one without doing a bit of research. Like any other market place, this is what you will find on eBay:

Since your goal is to learn how the watch works, then the only appropriate stock would be "in working order". You are buying a complete watch! It must have the case, bracelet, dial, hands, and the lot. Box and papers are not important.

It MUST keep time.

It goes without saying: any incomplete or water damaged Seikos should not be on your shopping list.

My personal recommendation: if money is not an issue, go for a brand new one! That's what I just did - I've just bought a SNK639 fitted with 7S26 movement for $65.99 including free delivery. Why? Because I already have a bunch of incomplete movements which I can use for parts. It will be a very nervous couple of weeks now till the baby arrives!

You are now standing at a crossroads (the first one of many to come!).

Both options are equally attractive - joy of sourcing discontinued parts or joy of wrecking a perfectly new watch :-)

Should you decide to go for a preowned Seiko then you must engage yourself in conversation with the seller. Try to find an enthusiastic seller who is prompt to reply and answer all your questions. The time keeping bit is crucial - it MUST keep reasonably good time (up to +/- 15 seconds per day. There should be no cracks in the case or crystal, no water damage and no rattling noise! But finding an enthusiastic seller who is going to waste couple hours on you for the sake of a $20 sale is always challenging. Squeezing a dollar or two is a waste of time, but watch out for shipping costs and especially delivery time. You want your baby in less than 2-3 weeks!

Again there is so much to choose from so don't buy the first one you see.

Model

Don't go for pre-1996 models like 7002, 7009 or even older 6119, 6023 etc. While those movements do look similar they are completely different and a DIY online tutorial will not be much good for you.

Also spare parts are incredibly difficult to source for those old models.

Read the title and description carefully and ask the seller to verify movement calibre- some sellers know less about their goods than you!

In case you insist to work on your own Seiko calibre then you can still follow DIY project but you won't be able to squeeze the most out of it.