I am also interested in having some precious metals in an IRA, but I'm not a big fan of the custodial arrangement that seems to be the only option. It seems like a license for the custodian to overcharge for silver purchases, and an invitation for fraud/confiscation with the PM in someone else's control.
I am a collector, not investor, so take it for what it's worth... If you want to collect coins, collect them. If you want to invest, do something else. I have been collecting coins for almost 40 years, and I couldn't tell you a sure fire way to invest. I am no expert, but I know that there are far better ways to 'win' at investing than coins. If you are dead set on metals, get something that sells for whatever the price is that day, no more or less... bars, junk metal, etc... Anything that has a premium due to collectibility is probably not a good idea for an investment you are counting on.
For all the people that i have talked with it is a bad idea to put your 401K into a Gold or silver account. Many of these accounts are over sold and the companies do not have the gold and or silver reserves to back them up. If you are going to invest in PM then just buy them out right. both are pretty expensive right now. you should not invest more than 10% of your entire portfolio in PM's from what i have read. as the motto goes if you don't have it you don't own it. all you get is a certificate saying how much you own. it is about as worthless as the paper it is written on.
In general, I'd say it's a bad idea to invest more than 10% of your savings in any one basket. That said, there are definitely times where it makes sense to have more than 10% of your money in stocks (with no one stock over 10%) or in PM (say, 10% in silver & 5% in gold, or whatever). It all depends on the situation and the price you can make the purchases at.