Pursuing a business that has your passion at its heart is always a tough choice. Working on something you love day in day out can be great but eventually it might kill your passion for the subject. Many people have come across this problem and then moved onto new business interests. Sometimes what you are passionate about is not what you might be best at.
Picking an industry you excel in is a key tool to any up and coming business. If your expertise lies in engineering you probably shouldn’t open a bakery even if you do like baking bread on the weekend.
Having the passion project on the side may be a better way to go. You can spend time on it when you get chance and treat it as more of a hobby rather than something that has to succeed. It is then more likely to grow into a profitable business and you can gradually spend more and more time nurturing it.
This may sound like counter productive advice as people will often tell you that you need to be happy in your work. Are you going to be happy though if your business fails and you are forced to work a job you hate to cover the expense?
If you have the time and patience starting a passion project it a great idea but in my experience it should never be your total focus unless of course that is where your expertise happens to lie.