As someone else who has reached, and somewhat surpassed the age where your eyes start going south, I'd highly recommend that you TRY some different options before you spend a ton of money. Many people do not get along very well with the BAC built into many ACOGs, I can shoot with one quite well, but my Brother can't use them at all. If you can use the BAC a 4x ACOG would make a great, if rather expensive, optic for you.
If you've got an astigmatism you probably will not like a red dot or an Eotech. Again, look through one before you buy. If you don't have an astigmatism, and most of your shooting will be under 200 yards then a good red dot or Eotech may be the best sight for you. They're fast and best of all, you don't have to be able to focus up close and at a distance at the same time to be able to use them effectively.
For longer distances you'll want some magnification. I prefer the low power variables with a minimum of 4x on the upper end and no greater than 1.5 on the lower end. I've got two like this, a Simmons and an Leupold. The Leupold cost 5 times as much as the Simmons but to be honest, I like the Simmons better and am going to be selling the Leupold to a friend.
Many low power variables also have a lit reticle and some are bright enough to be used as a red dot at close range. Most of these are $600 and above, but the Mueller 1x-4x that's in the works, assuming that it ever makes it to market, may very well be a great option, it should be bright enough for daylight red dot use and in the $300 price range. But to be honest, we've been waiting for it for a couple of years now. If I didn't have the Simmons 1.5x5x20 that I like so much I would have bought something else. But in another thread what's supposed to be the final prototype is being tested now.
Good luck and we're glad to have you back in the AR owners ranks!