Again, the question is what is it you want, and what scale? It could very well be that such a critter might have already been made in brass, and could be reposing on the shelves at a major model train store, e.g., Caboose Hobbies in Denver, Colorado. They have a large glass display case with a bunch of steam and diesel brass models in HO (and maybe some in O...I don't pay attention to other than HO).
I would NOT pay someone up front for a custom-built anything. A 10-20% downpayment would be the maximum I would put up. It is NOT that I don't trust such folks...though there have been ripoff artists out there. But even the most honest individual is a one-person proposition, and can be subject to the whims of fate, personally, family-wise or financially, and might not be able to complete the job. If they would take a credit card (which most aren't set up to do), it might give you some measure of protection. Even though it costs a bit extra, I take credit cards for my custom leather business. It is a protection for both customer and vendor.