The 600 seca belongs to a 62 model series of bikes collectively known as "XJ'S" as in xj600rj.
These are all great bikes! Once you put the time and energy into the regular maint issues they are near indestructible.
I also belong to XJBIKES.COM. there you will find EVERY thing you will need to know about these motors and how to fix/maintain them.
Before you trade be sure to check the Kelly Blue Book value. I bought my xj700n several hundred less then the asking price because I knew the real worth of the bike.
Besides compression ask when the last time the valves were adj,carbs cleaned,wheel and steeringhead/swing arm bearings checked and greased.
how about the brake pads and shoe? how old is the batt? these things will quickly add a ton of money fast if they need doing now.
Also....if you can do your own work it will be a big plus since a lot of bike shops won't touch these old bikes
Good luck!
Ron