Greetings and thanks for the question. I don't have a complete set of answers for you but I can help a little and steer you in the right direction. First, "officially" all Browning "Sweet 16" shotguns will say exactly that on the left side of the receiver. However, "unofficially" that's not exactly accurate. Over the years, some original "Sweet 16" examples have been refinished and that designation removed; other folks say that the US gov't bought some that didn't have that marked on them; and still other folks contend that there were "many" US made examples sold during the early period of production in the US that were not properly marked. I don't know exactly what to believe. I do know, however, generally the ones marked "Sweet 16" a priced higher. Now there are several other points that impact price and you mentioned at least one of them--grade. The new ones were graded, pretty much like the other Japanese produced Brownings. In addition to the various grades, There was at least one special edition version produced for Ducks Unlimited. If you're going to go for one of the Japanese higher grades, make sure you get the original box. The end of the box will show proof of the original grade. I had at least one customer who liked the A5s so much that he had them "upgraded" some by Browning and some by other engravers. For collectors and future increase in value, the real determining factors are age and condition. Generally, the older and the better the condition--the higher the price and the faster it will appreciate in value in the years to come. One quick way to determine the age is to look at the safety. The oldest examples have the safety on the front of the trigger guard that slides back and forth through the trigger guard. These were considered "unsafe" by somebody and replaced with at least two subsequent safety styles/placements. If you really want to know more and invest your money wisely, I suggest that you first invest in the book by Shirley and Vanderlinden, Browning Auto-5 Shotguns, The Belgian FN Production…
As you mention, there is a lot involved with the A5, but this book is a great place to start. There is also The Standard Catalog of Browning Firearms by Cornell. An even better source of information is the Browning Collectors Association. Even if you're not close enough to make their annual meeting, they have affiliated clubs where you can usually find other folks addicted to the A5 and/or the Sweet 16.
The Light A5's are generally limited to low brass shells, but even here there are some differences especially in the early production guns because of variation among US and European shotgun shell standards. Some folks will try to tell you that you can alter a "light" to shoot heavy loads by changing out the "piston rings", but don't believe that. You might squeeze off a high brass round or two, but not for very long. Forgive me for rambling, but one last point with regard to the older A5s--in that day and age there was no such thing as an "interchangeable" choke tube. If you wanted different chokes, you had two choices. The folks with money bought sets of barrels. So, you'll find the high dollar "Sweet 16" in cases with two, three or even four barrels. If you go this route, try to avoid keeping the shotgun and barrels in the case because the cases are know to attract and hold moisture, ultimately resulting in rust. The second option in dealing with the choke issue was the route the "common" folks took and that was the "poly choke". There were several manufacturers of devices that you could have stuck on the end of your barrel that you could adjust to the choke you needed. They weren't very attractive and aren't very desirable today, so consequently, when you find any A5 with a poly choke, it's usually priced less than a comparable one without it. These can be real bargains, especially if you're willing to have a competent gunsmith remove the poly choke and set it to accordingly or even install a set of screw in tubes if you're going to shoot it. Once again, please forgive me for rambling. Grandfather gave me his Sweet 16 along with the original sales receipt, so your interest brings back lots of memories. Best of luck and keep us posted.