Ok, I have about 775 hours in the game, so to me, yeah it is worth having, but different people like different things, so mileage may vary.
1) End game, well depends on how much you did during the main quest and how complete you wish to do the run. You can go slow and try to do everything before completing the game or you can rush it and do everything later, or a mix of both. Once you get done with your first run, they offer NG+, meaning you can turn around and run your same character back through the story, trying alternate things but starting at the level you ended the last game and keeping all weapons and stuff you had before.
2) Yeah, about that 100+ stuff......sure you can travel to them, scan them and move on...like the rest of the ME series, most planets are simply scan points for either resources or trivia about the universe. There are the main "hub" worlds, which if you have watched much on YouTube on it you have probably seen them, in which you will do your missions, and explorations. There are some really pretty worlds, but some people complain they are barren or lack diversity, and I can see that as the monsters you see on one world will generally be around on others...yeah there is a story reason for this, it doesn't bother me, but it does some.
3) Weapons go up to level 10, with augments that can be used during crafting...ie, you want a gun to shoot grenades, you can do that, if you want seeking bullets, you can do that, or you can go stock. You also have the add-ons you saw in ME3 of adding to melee, penetration, damage, and so on as a separate system. There is also crafting of armors, as well as using the crafting upgrades for them as well. The weapons basically unlock every 10 levels, with final one unlocking around level 80 or so...and since you should end the game in the level 60s range, then the only way to get the best guns is NG+
I am looking at the store now and for the discounted price I personally think the game is well worth it. To me MP wasn't ever a big thing, but I think there are even some people still playing it, and for the most part it is fun, it's just not something I will grind forever. I also think you need to remember that this is the first of a new series...hopefully....and not compare it to the ENTIRE Mass Effect trilogy, because you don't see the character growth and such when you compare it to ME-ME3 instead of only looking at ME1. I also think the devs fell into this trap too, trying to do too much instead of starting more restrained and detailed and opening up in later installments. But like I said, I have over 700 hours and have enjoyed it....
Personal recommendation....get it while it is cheap, and if you don't really like it, then at 75% off, you haven't really wasted much and if you get 2 hours of playtime off it, then it was the same as going to see a movie.....