If you're interested in Seiko divers try the following links:
https://archives.makedostudio.com/seiko-divers/ general info regarding all Seiko divers.
http://forums.watchuseek.com/f281/collectors-guide-all-seiko-7s26-0020-9-diver-variants-skx007-its-siblings-191450.html 7S26 series buying guide.
http://forums.watchuseek.com/f281/how-buy-seiko-7002-diver-collectors-buying-guide-258086.html 7002 series buying guide.
http://forums.watchuseek.com/f281/how-buy-seiko-6309-diver-collectors-buying-guide-140799.html 6309 series buying guide.
All series are very popular with the 6309 & earlier variants fetching ever higher prices while the 7002's remain reasonably priced. 7S26 variants are still available new - the SKX007K can be bought for around £150.
If buying older models beware, there is a lot of over polished, poorly serviced junk out there. As for the mods its really a matter of personal taste, some are good, some are bad, some are indifferent. Don't shy away from scrappers, they are good watches to hone your skills on, most of the original parts are available, although dials, bezels & hands are normally after-market.
Happy hunting.