I don't know about the AT65 (BT65?), but the AT44 QE Long will definitely be quiet enough, and a regulator should get rid of the ping as well, so VERY quiet. If you need something low power for the backyard, though, I would get something lighter and with less power. The AT44 is very long and heavy, and I DO enjoy shooting it, but for the task you have at hand, I would use my Brocock Contour. It only weighs 4.5 lbs vs. 8.6 lbs for the AT-44, and 16 ft-lbs energy vs. 38 ft-lbs.
As for regulators for the Hatsan, I would probably go with the Huma regulator as I think that is what bwalton uses when he tunes Hatsan rifles. I have also watched several videos (including the one by bwalton) and it looks pretty easy to do.
As far as scopes go, it depends on your scope budget, but I was just using a UTG 3-9x32 Bug Buster, which would work fine for back yard use. The rifle is large, so can accommodate larger scopes as well...I have moved my Bug Buster to my Brocock as it is a small gun and needs a small scope, and will be moving my Hawke Vantage 4-16x44 over to the AT44.
If you won't be shooting resting on a deck rail or table, you will probably want some shooting sticks or a bipod as the AT44 is long and heavy which is quite noticeable when shooting offhand.
My AT44 likes the JSB 18.1gr pellets best.
For cleaning the barrel, I just tie a patch on the end of some cotton twine, then hold the hose of a vacuum cleaner around the muzzle and feed the end of the twine into the breach. When the twine comes out the muzzle, I pull the patch through.
Wipe down your rifle with a gun rag (I just use a cotton cloth sprayed with a little Rem Oil) after each use as the oils from your hands can be corrosive.
