Treks are known for being pretty dependable well made bikes, even the lower cost ones.
If that Trek fits you ok it might be a fine bike to get started on- likely better and easier to ride than the old heavy one you are on now. Plus, it's new! You can always sell it later.

Don't think about biking to lose weight. Think about biking to get a healthier and more active lifestyle started, and to feel good all over. Weight loss will follow along naturally if you ride a lot. Bike riding is good for body and soul.

First things first....regular little rides of 5 or 10 miles... and learning to enjoy riding your bike. Keep it fun and not a chore and you will more likely want to ride more often. Welcome!