haha,
@rottenruger I think you were far more impressed by that statement then you should have been. It's true I shoot a mile on occasion, however the targets I'm hitting are quite large. If someone says "I can shoot a target at a mile" that's not really a statement of skill, throw in a target size and now we're getting somewhere, throw in how many impacts you get per 10 shots and now we really know what's going on.
So, the target size I generally engage at that distance is a rock somewhere around 3 feet in diameter, give or take 6" (2 ish MOA targets are what I look for to shoot at) and just estimating I hit 50% of the time at that range, maybe less. Without wind probably 80% but with wind probably 30%, just estimating off the top of my head, more wind, less impacts. You hitting a 6" plate at 500 with nearly 100% impacts is actually more impressive than what I'm talking about. There are additional factors at those longer ranges, but nothing a good ballistics solver won't help you overcome.
The main point I and I believe
@clcustom1911 were making on magnification is that if you zoom way in, sure you get better resolution, but you lose the ability to spot a miss if your recoil knocks you off target and you can't find your target again before the bullet strikes. If you zoom out a ways when you fire, your target will still be in sight and you can watch for your bullet strike then make appropriate corrections. One other huge advantage of a lower mag scope is you tend to get more elevation adjustment.
To answer your questions, I'm 35, have 20/20 vision and use a sightron SIII for this shooting ($800ish scope), Seekins rings on 20MOA scope base (6 creed or 6.5 creed usually). I, like you, have a fairly limited budget so try to find "high value" items, the SIII seems to fit that mold. It's the 8-32, but like I said before I hardly ever use the top half of it, even at long range. Ironically I use the higher magnification more at 100 yards then at long range. I think as far as aging eyes, or any eyes really, clarity always trumps magnification. You obviously need some magnification, but I wouldn't make that a primary concern, your Vortex and Sightron you already have will do the job I'd guess. There are guys who shoot fixed 10x scopes at very long ranges successfully. We're talking about hitting steel here, if you want to make a tiny group at 1000 yards, then you need the high magnification to have the resolution you need to make sure your aiming point is as close to the previous aiming point as possible. And in that case you're not concerned about spotting a miss (I don't think).
My guess is you'd be more successful shooting a mile than you think, and that buffalo. Don't talk yourself out of trying it simply because you don't think your equipment is expensive enough. It's easy to be intimidated by not having the best stuff, but someone will always have better gear than you, get out there and shoot anyways. Sounds like your biggest limitation is opportunity to shoot that far. See if you can find someone with land who has rocks you can shoot at, or varmint mounds or whatever.
Thank you for your service to our great nation sir.
