Buying coins as an investment is a little different then buying coins as a collector. As an Investment, I would shop around and buy the best coin I could afford. As the saying goes "the most valuable coin collection in the world can be held in one hand." This can be translated into buy the best date possible to buy the highest grade possible of a non key date. This in my opinion is where the fun starts when you start comparing prices and grades and figuring where you can get your best deal.
As a new collector, I would stick to US coins (unless you are from another country and then maybe tour the local coin market). As an Investment I would also make sure anything I bought was graded. As far as grading I do not think it matters on lower grade key date stuff as any of the big four companies can guarantee authenticity. If however you are buying a high grade coin I would suggest PCGS or NGC only, for this one reason once you are ready to sell it for profit these companies are recognized as the Top experts in the field.
I would suggest staying away from raw coins as a new investor.
Cheers - Galen