Well I not an expert armorer, but I am an engineer and have studied materials some while I was in school.
M1a's have had cast receivers since they were commercially introduced. The key to remember is that casting has improved a lot in the last few decades. I have never heard of a cast receiver becoming wore out.
As to the advantages used vs new M1a's I would consider the following.
1. Price, a used national match rifle should cost less than a brand new one, unless it has been customized or build by a renowned armorer. The difference in price can be used for ammo.
2. USGI parts are more common on older rifles.
3. I have heard that certain Springfield armory receivers have had scope mounting issues. But I don't think that this apply to ones built in the early 80's.
4. For price I would say expect to pay about 80% of what a new NM M1a sells for if your looking for one in good shape. Judging by the my experiences in Indiana I would say a nice used NM rifle will cost around 12-1300, unless it has been customized or build by a renowned armorer.
Hope that helps.
