I think you could make it with Construct 2, but if I were trying to make it, I'd try doing some simpler things first. Best to vent your learning frustrations on projects you don't really care that much about.
Also, if you do a number of simpler projects to begin with, you may end up finishing your big project sooner because you won't make so many false starts.
Re your questions:
1) Selecting objects - All of this is straightforward with Construct 2.
2) Gravity - You can change gravity of objects that have the physics behavior and objects that have the bullet behavior, but it sounds more like you'll want physics. You can combine behaviors, but sometimes you have to disable one behavior and enable another or things go sort of haywire.
3) Weight - you can create an instance variable on an object where you store the weight you assign it, perhaps based on some formula. Then the scale would make use of this to show the player the weight.
None of this sounds hard to me, but I've done quite a bit of experimenting. Check out the tutorials section and Kyatric's
How Do I Frequently Asked Questions
In particular, do the first two tutorials (Ashley's Beginner's Guide to Construct 2 where you create the Ghost shooter and Velojet's tutorial on making a platformer.) Even though you might not be building one of these types of games, they'll help.
Also examine the Examples that come with Construct 2. You may find the physics examples interesting, but it's worth going through them all, IMO.
Eventually you might take a look at Sqiddster's How to make a gravity based platformer. Certainly go and play the game on the arcade (under Rotary Games, I think) to see some ideas about working with gravity.