The main advantage of Delphi is that programming with it is much more effective than programming with a C/C++ environment because Delphi has the fastest compiler on the planet (and therefore the shortest build-times) and the code is so nice and clean, it will help you avoid many potential bugs. (Therefore Delphi is really great for game development).

As for the graphics engine: I've played Monkey Island 4 and I don't think you will need much of an engine at all so you might as well write it yourself. All you need is to be able to import and animate 3d models of some sort. Write an MD2/MD3 loader (there are plenty of examples on the web) and you basically have your engine .