Am I taking a risk?

I would. You shouldn't have any problem other than the battery might be low if it hasn't been used in a while.

In my opinion you are taking a risk buying anything with boards in it that has been sitting around. It seems that sometimes these electronics have a shelf life. Other than that you should be ok. Another thing you can do is make sure you can get replacement or upgraded boards from MTH before you make a offer.

I fully agree with Chris, the worst you could face is a dead battery, in fact, I suggest you buy a BCR and fit it. Don't forget, you will have the old 5 volt system, so don't fit a battery or BCR for a 3 volt board. Yesterday I bought a new/old stock gold plated Millenium Hudson, it's PS1. It will run on rollers in a display case in my shop.

Just received a proto 2 Pennsylvania P5A made in 2000 without any problems. So buy it, should be no problem. 

3 volt board came about in around 2004. If a 3 volt board goes some of the componets can be replaced. Not so on a 5 volt board because of the way it is put together.

If a 5 volt board goes it can be replaced with a 3 volt for the price of of the board around $150 plus around $35 for a conversion 5 to 3 volt board. Minimal labor cost.

You should be OK with the 5 volt board. There is never a reason to replace a board that is good unless you have money to flush down the toilet.

There is always a risk.  There are lots of horror stories about what is in that mint/sealed box.  In most cases that doesn't happen but it is a risk.  I forget what the exact warranty cutoff is if you are getting it from a dealer but something that old would doubtless be up to you to pay for repairs.  I'm not saying not to buy or that Chris and Dave are wrong.  I am just saying that you could end up really upset if you assume there is nothing that could go wrong.