It's a fine choice.
You will be able to get 4,000 rounds of 5.45 ammunition for the price of 1K of decent .223 ammunition.
5.45 is available as Bulgarian or Russian surplus. They are packed in sardine cans which have a very long shelf life. Wolf makes a commercial version (i.e. ammo that is not gov/mil surplus) that is non-corrosive. Otherwise EVERY KNOWN ROUND for the 5.45 should be considered CORROSIVE. This means the primer has a chemical composition that deposits salts on the inside of the bore, which attracts water and will eventually rust the bore.
This salt will not dissolve with gun cleaning solvents; you need to use hot soapy water or better yet, run some Windex-soaked patches through the bore. The ammonia dissolves the salts. Make sure you dry thoroughly and oil the bore.
I don't honestly know if the S&W guns have a chrome-lined bore or not. If they aren't, you ABSOLUTELY must perform the above servicing after even 1 round fired. IF they are chrome-lined, consider the above somewhat optional but a very, very good idea. Guns will rust in as little as 12 hours in a humid climate.
The armchair tacticians will tell you how horrible the 5.45 is as a man-stopper; how the velocity is low and it's not much better than getting hit with a spitwad. Nonsense. I dare any of them to get shot by it. I have found the milsurp 5.45 to be very accurate out of my AK's. I haven't shot it from an AR.
As you pointed out, the versatility of the AR15 platform is the interchangeability of the upper receivers. If you so desire, you can get a "mil-spec" upper receiver and put it on your lower receiver and with the proper magazines and buffer choices shoot .223, 6.8 remington, even .50 caliber rounds. With a bit more modification you can run pistol calibers. Hell, there's even an upper that takes the FN 5.7 round with 50 round magazines. All of these will work on your lower receiver.
If you weren't aware, the lower receiver is considered the firearm. It has the serial number. You can still buy upper receivers without any registration or restrictions. Once you have a lower receiver, you're good to go.
I can guarantee you that Obamination will restrict the importation of ammunition. The first to go will be "non-sporting" ammunition such as milsurp ammo. That means your cheap 5.45. You MUST get online and order as much 5.45 as your credit card can handle. It will NEVER be cheaper than it is now. Ever.
You will be always be able to get at least what you paid for it, and as time goes on, much more. If I had more space, I'd be stocking up on it. I have about 15K rounds right now. I shoot it in my post-sample 5.45 Bulgarian Krinkov. It is MOA accurate (proven), reliable and fun to shoot (obviously). Plus, at 8 cents a round, it's almost .22LR cheap.
So, to make things easy for you- BUY THE GUN ASAP. YOU MADE A FINE CHOICE. IF you don't like 5.45, I'll trade you out uppers.
Oh, forgot to add- buy a crapload of magazines as well.
or two
I'm sure you'll be okay with the one or two you have.