Cool costume! Sounds like it's gonna be wicked! What all have you made already?
The hardest part will indeed be the corset because to make one well takes time. The only corset I ever made is three layers with lumber strapping for boning. It took 3 days of near-constant work (but I was really amateur at garment making at the time). The good news is it squishes me with hardly any wrinkling, so it turned out very sturdy. I used Truly Victorian 110 http://www.trulyvictorian.com/catalog/110.html
. It was pretty easy to grade the pattern to my measurements, and only took 4 business days to get to me after ordering. If you went and bought one of the Simplicity corset patterns, make it 2 sizes smaller than what it tells you. I got fed up with that pattern after 2 muslins and abandoned it for TV110.
I'm pear-shaped too! For balancing out your look (especially if you decide to cinch with a corset), I'd say a bunch of ruffles up near your face would help alot. Do you have alot of problems with pants fitting in the hips but the waistband being too loose? I hate that so much, but if you're making the pants from scratch then you'll totally get to avoid that.
Maybe to save a little time, buy a second-hand blazer bigger than your size and butcher it according to your intended look/pattern that you have. I absolutely adore what this girl did with a second-hand suit: http://www.craftster.org/forum/index.php?topic=212286.0
Oh, and maybe add fingerless leather biking/driving gloves. A cameo or floral brooch on the top-hat would be cute, too.