If the seller in a stock sale agrees to terminate all of its ERISA plans PRIOR TO CLOSING and take responsibility for doing so in compliance with ERISA, Code and other applicable laws, does the buyer have any obligations with respect to the plans?

I assume that the buyer needs to make sure it doesnt have a successor 401k, anything else??

If it is a stock sale, then the buyer of the stock steps into the shoes of the owner of the shares. Any obligations of the company continue. The Tax Code, ERISA and economic consequences of those obligations will now effect the share value of the new owner.

So, in a stock sale, it is typical to cover these risks in some sort of contingency agreement - holdback, etc.

This is just one of many reasons why buyer's tend to want to buy assets, not stock.