I have a 1991-A1 Commander I have not messed with in a while and was looking to install a threaded barrel on it for a suppressor. What is a good choice? I would need to get suppressor height sights as well. Is it a good idea or just keep it stock?

1991, as in Colt Series 80? If so, they have a narrower barrel hood than Colt's Series 70 or GI guns, so whatever you get, odds are you'll be needing to do some work to make it fit correctly in the barrel hood area. The only one I can think of that's readily available for Commanders are Ed Brown barrels, but I'm sure Jarvis, Barsto, etc will make you one...the Ed Brown will for sure require some fitting, but it's not too bad to do.

What kind of sight cuts? If it's the original front sight style, you're better off sending it to Novak and having them cut and install sights...I believe they finally came out with a set of suppressor height sights, too.

I don't want to alter the gun in any way really. It has standard sights. I guess I can just keep it stock and buy a threaded barrel for my G21. It's in MINT condition with the original box, mags and paperwork, hell I even have the original sale receipt.

...they're not all that collectable though, unless it's a super rare one-off or something? Commanders, 80 series, are a dime a dozen and are still made today...I wouldn't worry about collectibility/future value unless there's something unique about it?

Sormlake used to make a decent quality drop in,  since discontinued, but they pop up for sale now and then. How drop in is "drop in" ? impossible to say until you try- mine works great on my Guncrafter, Kimber and SA 1911's except my SA Pro- It wouldn't work in my Nighthawk or Baer either.

Don't bother with suppressor height sights, unless for some reason you're trying to do great groups at 25 yards.