Hi Kaleb,
I would start will a simple Osc engine from bar stock. If you make a mistake you can just do it over. If you make a mistake on a casting engine you'll have to buy a new part!
I'm with Tony especially if you're just starting out. I've built several barstock engines and just completed (nearly) my first casting engine. The casting engine was from PM Research and I did end up ordering an extra part that I couldn't salvage from the kit. On the other hand I have a box of "extra" parts from my barstock engines, probably enough to make an engine ;D.
That said, many folks have started out with casting kits and had no problems. If you're the type of person who wears out a pencil that still has a full eraser then a casting engine might be right for you
In any event, good luck and success in your choice.