I think it is more likely you'll get a common Gonodactylid or an O. Scyllarus though. $37 is a great deal if you do get a Peacock. If I were you I'd put in a request at blue zoo for a small one. They are really good about getting smaller size Peacocks, but they are indeed expensive (especially with shipping).
Having had my Peacock in a 40B for around 7 months now, here are some thoughts: I think the 40B w/40B sump idea would really work well for any mantis. Get a plant growing bulb to put in one of those cheap home depot light fixtures and you'll grow piles of Chaetomorpha. You'll never have a nutrient problem, especially if you run a skimmer with it. One observation I have made though is that even a 40B looks too small for a Peacock sometimes (at least to me). They just roam so much. If you watch dive videos, you'll often see people following Peacocks around on the ocean floor and they roam for LONG distances. If I ever upgrade, I'm going to get a custom built glass tank with a huge footprint, maybe up to 6' long. I'll keep it relatively short (14-16" tall) and maybe 16" wide. A 72 x 14 x 16 tank is 70 gallons and I think I could manage that.