Custom chassis is the ONLY way to go for the Merc 12, there will be a few expenses to make that work:
Transaxle, there are only a few that can take the torque Cima, gearfox, Graziano, Ricardo, G50&G64 porsche expect 10-30k
Engine Management, megasquirt, haltec, autronic, electromotive $1200- $5000
Headers, Ive seen $500-$5000 for a custom stainless set
Having said that, welcome to the club you will find that you can get a ton of advice here, be willing to ask questions and more importantly willing to listen to what is being said