You do not need an actual working model of your invention when you submit the application. There is a requirement for "reduction to practice". Reduction to practice can be "actual" (a working model) or "constructive". Constructive reduction to practice consists of describing the invention in such detail that a person of ordinary skill in the art could make and use the invention.
So if you have a good understanding of how your gadget would work, and you can teach that mythical person of ordinary skill in the art how to make and use the gadget, then yes you can apply for a patent now. Of course if you wait until you have a working model, then you will be able to describe the gadget in more detail, present more alternatives, and perhaps have a more robust patent application. Or you can do both, apply now and apply later.
Each time you file a patent application it is going to cost you time and money; so unless you have a lot of time and money, you need to develop a filing strategy. A patent agent or patent attorney can provide alternative plans, and pros and cons of each; but it is your business decision to make.
About an engineer stealing your invention, it could happen. Most patent attorneys can draft a contract between you and the engineer that would protect your interests. You would require a confidentiality (or non-disclosure) agreement, and an agreement that assigns you the rights to any new inventions that are developed as a result of the work the engineer does for you. Whether it would be difficult to find a qualified engineer willing to sign such agreements, I have no idea.