Your questions are a little too general. For shooting big game, 20X is more than enough. Just a couple of days ago I was sitting up on an ambush spot with a view of ranges from a couple hundred yards to about 1500 yds. So I decdided to do some "scoping" wiht my NF 5.5-22 set on different powers at different ranges. My conclusions.... 5.5x through the NF is more than adequate for deer size or bigger game @ 1000 yds.
Now when shooting paper with small bullseyes. I like the full 22x. That bieng said, it's possible to get by with much less. For PDs and varmits, 10X or greater would be very nice @ 500 yds.
There are different types of steel and it depends on what bullets you are shooting out of what rifle at what range.
If your going with a standard A36 type steel then I would think that 3/4" should be fine depending on caliber. If your going with an A500 or a mil-std armor plated steel you will be able to go thinner. For what its worth a 30.06 goes through 1/4" A36 steel like butter at 200 yds.
I have an 18x Nikon that works great at 1000yrds. So 20x would be better for me for more precision, but for 500 you could probably start counting hairs. 10x to 18x should be sufficient. Like another said, if you are shooting bench then all the power you can afford would be a good option. Clear is good!
One thing to consider regarding the power of the scope is if you are using it to determine the size of the horns or if it is a buck or a doe. I know some people that have shot does at 300 yards at 9 power,but when they reached the animal it was a small buck. I much prefer the higher powers of the variable power scopes.
20 power is lots, I have the same scope, really like the glass and turrets just not enough adjustment for me, I also have a 6500 elite and a nightforce. As far as steel goes it depends on caliber and speed and how the steel will be placed. If you let it hang so that it will move you can grt away with a lighter material. We have a peice of qt 100 at a 1000 yards that we shoot with .338 edge , .338 edge a.i. and .338 lapua , its is 1/2" thick hanging no problems 4 guys shooting it.
A lot depends upon what kind of hunting you are going to do and how many deer you are going to let walk and why you are willing to let a deer walk.
Yesterday with a 5-22X-56mm NF scope, it was set at 12-13X to shoot a deer at only 260 yards. We were on a flat trying to shoot a deer that walking along a hardwood forest hillside eating acorns. That was the magnification that was needed to identify limbs that would deflect the bullet. I was on the spotting scope but my line of sight angle was not the same as the shooters and so I was not the person making the decision if the deer was clear of limbs. That had to be done by the shooter using his scope. On the first occasion no shot was taken because the risk of deflection by limbs was simply too high. On the second occasion the shot had to be placed over a limb that was blocking the lower part for the deers chest. This is very much precision shot placement requiring the same type of magnification as competition shooting (in fact the rifle was a 40X used mainly for competition).
Based upon hunting deer in the east and antelope and elk in the west I would say a good quality variable scope with a top end of about 20X will satisfy almost all of your needs out to 1K.
Its amazing the amount of effort it takes to pick the correct scope. One that fits your needs without emptying the back account. I've been reading through forums, online reports, and manufactures info and I'm still unsure of which one to purchase.
I have done a lot of long range work with a 10X Shepherd. I have used it to make hits on milk jugs at 1,630 yards. I was able to take a bull elk at just over 1,000 yards with it this year as well. Now, I want and definately feel it would be beneficial to have around 20X for those kinds of distances. It's being able to put the crosshair right on that "spot." But amazing things can be done with low magnification but it just requires a high level of concentration. I would think at 500yds 10X to 14X would work great.