With the prices of new ones, why not buy a GI carbine ?
Well, there are a few good reasons not to buy a GI.
1. If you want the original configuration of the flip sight, push button safety and no bayonet lug. I can't give any real historic data here, but in my experience most GI carbines will no longer be in this configuration.
2. If you don't want the hassle of inspecting a used carbine and any issues that might arise from one.
3. If you want to "modernize" it in any way. Really all this means is putting a red dot on it, or putting it in a new stock, so it's not like you're actually going to bubba the rifle, but you aren't going to keep it in its historic form.
I don't see much merit to a new stock but a red dot is quite useful on the M1 Carbine because the sights just aren't that great, especially if you have the flip sight. If you want to use the M1C for deer hunting, your ammo may not align with your POA.
The options for a dot mount include Fulton's, which replaces the rear sight, the Amega scout mount (which would be very difficult to find these days), and the Ultimak scount mount, which might be the best option because if your dot fails you can remove it and still use your irons. I went with the Ultimak for my Auto Ordnance carbine. And frankly, the M1 Carbine actually looks really good with this setup and the mag pouch on the stock. It is an incredibly light, handy option for deer hunting in New England (you'll need a 5-rd mag in NH).
Of course, you can certainly do all this to a GI carbine, especially since none of it is irreversible.
The Underwood load has been mentioned - that's a great round. However, I chose the Federal Premium 110gr Power Shok JSP. I find that it pretty much hits exactly to the same POA/POI as 110gr FMJ. Buffalo Bore also makes a 110gr JHP "Full Power+" load as well as a particularly nasty 125gr hard cast, both at 2100fps.