Rollcage kit?
I am building my own. (not that it helps you) but I would like to say that if your not careful you can chew up alot of tubing. We estimated 110 ft for my car (for a 10 point) and it has taken us about 210 and we are not done. Now this is the fisrt cage I have ever done and I have screwed up a bunch of tube trying to get it right. And a pro should not have issues like I did. But keep in mind you might be looking at buying more tube than just what is in the kit.
now for cromo vs mild steel. Both are acceptable. Cromo is lighter but requires a TIG. MS Is a little heavier but can use TIG or MIG. Cromo is only lighter because it's stronger and you can use a thinner wall without losing strengh.
on an 8 point roll bar there is no time limit on it's life. It will be good forever, a 10 point (asuming your faster than 10.50) will require certification, and it's good for 2 years. (then you just have it done again and your good to go) just keep the car slower than 10.50 and you won't have any problems.