Where are you planning to establish the corporation? If in the U.S., you can incorporate a business or simply open a shop and apply for a Class 1 FFL with a Class 3 Special Occupational Tax. You don't really need a lawyer for any of that (though I recommend it). But the problem is that machine guns can no longer be purchased by individuals, and even if your licensed "corporation" buys some, they have to be disposed of to a government entity or another licensee if the corporation goes out of business; in other words, you can't just start a corporation, buy guns, fold the corporation, and keep the guns.
As to buying guns in the US and exporting them to Australia, I don't know what the laws are in Australia, but I am under the impression that privately owned MG's are illegal. If that is not the case, you would need a lot of paperwork to export them to Australia. The BATFE enforces the NFA but does not control exports of arms and ammunition, the Department of State does, and Australia would have to approve the import before DOS would approve the export.
All in all, unless you really plan to make a lot money on the deal, the task is daunting.
I think you need to think things out before being concerned about a solicitor.
Jim