Sorry, I got tied up in the numbers and neglected to comment on your question regarding mounts.
If you add any sights - receiver or scope - you'll have them back up on your receiver unless you take precautions. There are shallow holes in the top of the receiver that acccept studs on the proper mounts - this combination prevents rearward slide of the scope mount. You'll need a separate stop of some sort for receiver sights, and you place your sight in from of it. Sorry I can't recall the proper terminology - it's been 10 years since I bought something for it. They sell those, too.
I bought the $35 receiver sight with the rifle and mounted it myself, without the stop. (Mistake. It walked back until it used the safety as a stop, rendering the safety unusable.) You can do this pretty cheaply and add the mounts and scope later if you want to. The people who sell you the rifle probably have the stops, too, and they can't be too expensive - $5 or $10, I'm guessing. One thing for appearance might be a piece to fill the holes that appear when you remove the rear open sight on the barrel - I never did, but just haven't got around to it yet. Maybe in anther 10 years.
Envision the geometry if the break barrel doesn't point directly in line with the receiver. No problem with the sights mounted on the barrel, and they are pretty good open sights. If they're on the receiver, you have a different issue. Most barrels point slightly down. I suspect that's so you can crank the sights up to zero with the point of impact. If the barrels were to point up, you'd soon run out of corrections down to zero it. The next Beeman I buy will have the "barrel angle" option (a few extra dollars), because I'll definitely be using sights on the receiver, peep or scope. That being said, my Williams 5D sights made for Beeman are reasonable quality and had all of the "up" correction I needed.
For the Simmons Air Rifle scope with Adjustable Objective (good quality, I think), I went with an adjustable scope mount from B-Square. With it, I could spin the rear up to compensate for the down barrel without excessive cranking in clicks of adjustment. The rear part also had a stop that fit the depression in the reciever, so it's on there solid.
One other thing. The grooves in the receiver are 11 millimeter, rather than the standard American standard 3/8-inch, so make sure you get the right size mount or sight. (The grooves are more different than just the width - their angles are different, too.) No big deal, just be sure you get the right ones for your rifle.
Jaywalker