Hey Walter,
Oh the fun, technology that just keeps working. It's hard to replace something that isn't broke, right?
So here are the easy answer:
SATA is SATA, so any 2.5" SSD should work fine in your computer, all the way up to the 4tb models.
1) SSDs can be more reliable, especially in machines that are moved around. As your desktop isn't that, I'd consider it equally reliable under normal use. Hard drives have moving parts and they will fail. SSD's have chips that will wear out. Both can be total jerks and die at any time. Backups are important.
2) Yes, working with files on SSDs is much faster than working on HDDs - between loading and saving, or just large files, there is a noticeable difference.
3) SSDs tend to be less warm than HDDs since there isn't a motor spinning things around. 5400 rpm drives are cooler than 7200 rpm drives which are cooler than enterprise SSD's at 10,000 rpm.
The thing I would do is consolidate the drives in their current state into a single larger drive and a bigger backup drive. Drives that are 1tb & 2tb are too small in my opinion in this day, and that moving to a single 6tb drive would be more efficient & reliable. If you want to go a bit further get 2 and run a simple mirror for redundancy. Your backups are important, but you should be best in 1 SSD for boot/Windows, 1 SSD for active photo projects, 1 HDD for photo archives. Toss in a set of backup drives and you'll be good for a while.
-Joe