If this is like purchasing any normal software house, you need to make a direct offer for intellectual property and patent rights to Mr. Hitech aside from the outright purchase of his company.
Without those he can sell you the current business entity and code while taking his genius else where. $5Mil sounds about right for that excluding the patents and genius. Who knows, he may have one more competitive venture in him that will migrate your customers away from your AH just like they did from War Birds to his AH. He has a very talented history of creating games and customers following him to newer better games. You could never pay him personaly enough to stay on as your coad slave while giving away his genius for a paycheck I will venture.
Why do you think he would?