SQLServer get top 1 row from subquery

You are going down the path of using outer apply, so let's continue:

SELECT P.ID, P.Description, P... blah, blah
FROM Products p OUTER APPLY
     (SELECT TOP 1 B.Product,B.Date,B.Price --Can't take out TOP 1 if ORDER BY
      FROM Buys b
    --WHERE... Can't select by P.Product because doesn't exist in that context
      WHERE b.Product = P.ID
      ORDER BY B.Date DESC, B.ID DESC
     ) buy
WHERE (Some product family and kind restrictions, etc, so it processes a big amount of products)

In this context, you can thing of apply as being a correlated subquery that can return multiple columns. In other databases, this is called a "lateral join".

Seems like a good candidate for OUTER APPLY. You need something along these lines..

SELECT P.ID, P.Description, P... blah, blah
FROM Products P
OUTER APPLY (
    SELECT TOP 1 B.Product,B.Date,B.Price 
    FROM Buys B
    WHERE B.ProductID = P.ID
    ORDER BY B.Date DESC, B.ID DESC
) a