Good advice here. The difference in 45 to 50 may seem small, but it is more than enough to cause form problems. If Mike can fix you up with a Dryad for about the price of a Montana, I'd jump on that! But, I'd also go the used bow first route, then get the Dryad at the weight you think you want to go with. Get a used 40-45 pound bow, then save up for the Dryad. As George said, there are a bunch of lefty bows in the classified section here. I haven't looked at the auction lately, but they probably have a bunch too. Given the choice, I'd buy from a Tradgang member before going to the auction. There are good folks here, so you are less likely to get burned.
Oh, and welcome aboard! You have found the best source of inspiration and information in the world for the traditional archer.