Congress, sure, but it was the president who authorized the purchase. There wasn't even a law about buying property. Necessary and proper is completely irrelevant, because there are no laws involved.The Constitution provides, in Article 4, Section 3, Clause 2:
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States;
This would imply the power to acquire property (with Congress not necessarily being the party to do the acquiring). Clauses dealing with treaties, appropriations, and the necessary and proper clause could also be seen as supportive of the Lousiana Purchase.