The DP helmets are the most commonly used in this forum since the beginning, and is the cheapest way to start your costume. The '95 DP helmet is the most recommended of the series, for it's shape and quality (not the most accurate of helmets, but it is from all the DP/Rubies).
The MSH and other helmets were created afterwards by members of this board and are quite expensive, depends on your money-value point of view. These are in the range of $100 in kit form to $300+ completed and painted. All you have to do is to scour the site yorself and you'll see the answers you're looking for...just be patient.
Robert