Welcome to the forum, YukonPete. Are you by any chance related to Yukon Cornelius?
If I had the suspension in hand as part of the deal, I would go ahead and install it.
I would guess the backfiring on deceleration is more a function of your air/fuel mixture. Perhaps somebody who uses the Supertrapp muffler can guide you in adjusting your mixture screw?
You really can't tell what you've got in the way of doohickey components and their condition unless you open up the left side of the engine and look. I think most would advise you that if you plan on keeping the bike long-term, you should at least open it up and check out the condition of the spring. Most agree the Eagle Mike torsion spring/lever upgrade is the way to go so you never have to worry about it again.
Since you have no way to tell if your valves have been checked/adjusted, I wouldn't worry about it much until it's convenient for you to check them. If you notice the thing is taking longer to start, that's a symptom the valves are becoming tight. Were it me, I would just plan on checking them during the next off-season to see how they look. In the meantime, ride on.
Some KLR's in this age range had some issues with the wiring harness routing and rubbing that you might want to check into, especially if you're looking at longer or more remote rides.
All that being said, I'm more of a KLR Stooge than a KLR Guru and I would refer you to this excellent thread, written with those with more knowledge than myself. It makes for some good reading:
Confidence riding on dirt and gravel is purely a function of hours and experience. Take it easy until your experience/confidence builds. Roads like that are what the KLR is made for.
Again, welcome to the forum and enjoy your new ride!