Is a diesel engine a good project for a complete beginner DIY mechanic?

Either way, you are going to get your hands dirty. Good for you. Doesn`t matter what others say, It`s what you feel comfortable with. Don`t be dissapointed if you don`t get it right the first time, just don`t let it happen a second time. Start with a lawnmower motor that barely runs and make it sound like new. It has all the basics, and if you mess up its easy to fix. But whatever you decide Have Fun.