the 007 has a much wider range of products to be modified, something very important to modify is where you want to take the watch, keep the "diver" concept or change it, and This is where the modification will start because when changing the concept you must change the dial and the hands of the new concept. (diver, pilot, GMT, casual, Samurai, etc ..) In addition to the improvements in the crystal, crown, caliber if you wish etc ...
personally what I like a lot about Seiko is its case steel, it is excellent, very hard and you can really get a "mirror" shine when you polish it, unlike other brands where I have seen its work very poor and soft (prestigious brands)
Now as to whether it is J or K, the most important thing is its precision, the accuracy with which it can count every day, I have had in my hands new calibers made in Japan where they are very out of date and I had to adjust the precision in them but in general they are gauges very easy to adjust and repair or replace,
The truth is that in some modifications if you can get to spend more than the watch itself can cost, but believe me the change is worth it
Regards
Rene