SV is not comfy for longer distances, more of a forward lean that puts stain on the neck shoulders, back, and wrists.

Pre bikes have lightly more power, as post kats had some changes due to the EPA. Looks wise they're clearly different, but otherwise they're very similar. Parts aren't necessarily interchangeable, but they're built the same way. Both are heavy, have a longer wheel base than most other similarly styled bikes, are cheaply but well built, reliable, and pretty easy to work on.
Post kats probably have slightly greater parts availability, pres haven't been made in almost 20 years now.

For 2 up riding, if you get a kat the suspension will need some work. It's meh for solo riding at best, but change a few things and it's great. I'm not sure how that compares to the FZ or ninja. My buddy has an FZ07, if its similar to the 06, it might make a great bike for 2 up. Already comfortable, already handles pretty well. I love my kat and wouldnt trade my buddy straight up if he offered even though his bike is 8 years and 100k miles younger. But the 07 is a very good bike. If possible, see if you can take both for a test ride. That will probably make your mind up faster than anything anyone tells you.

What kind of suspension work are we talking? Adjusting the suspension or completely replacing the suspension?

Thank you for the input. I have definitely considered an FZ-06, but the ones I have seen are generally around 1k more expensive than the Katana

Upgrade the front springs for your weight with emulators and install a reworked 750 shock with a spring for your weight. Like night and day. Terry can set you up...here is his website http://norwestsuspension.com/

New springs in the forks you can do yourself, $120. Buying a rebuidable shock and having it reworked will likely be around ~$500.