For starters, as a longtime M1A owner, let me start by saying that Sharpshooter's advice is right on the money here.
I think the "loaded" M1A offers the best combination of features at the most reasonable price point. If you buy one of the base rifles, you'll probably spent a lot more than $500 on separate upgrades, and there is no guarantee you'll be pleased with the work. You're risking a lot of aggravation and inconvenience down the road. With the "loaded" models, you know exactly what you are getting up front, and they seem to be set up quite well right out of the box.
Of course, I also agree with Sharpshooter in that a Supermatch is probably
not worth the extra expense unless you are a regular at Camp Perry. Rear-lugged receivers are the Holy Grail of competition M1As, but you could probably shoot a standard NM rifle for a lifetime and never realize the difference. It's a competion thing, and unless you're planning on dropping a lot of money into competition equipment and training, this is money better put into ammunition.
Walnut or synthetic? I say walnut, and I already know that I am dead wrong. You simply can't beat a McMillan sythetic -- it's THE perfect rifle stock, bar none -- but, I'm a bit sentimental, and I just
like walnut better for 100 reasons that probably don't hold any water at all.
One thing is certain, I dare say that nobody ever bought an upgraded M1A and walked away disappointed. They are the "rifleman's rifle."
Chuck