A lot of this is personal opinion so I will try to be fair while telling you what I would do.
External Overflow boxes give you more room in the tank but also mean the tank needs to come out further from the wall and can make the plumbing under the box difficult to work on. Additionally I have had tank manufactures tell me that they believe an internal is more structurally sound long term than an external.
Unless you must have the external for aesthetics, I would go internal. Better structural reliability fewer potential locations for a leak and you can back the tank against the wall like most people want to. If I had a fish room behind the tank I would go external because I could better access the plumbing and I could cut through the wall so the tank could still sit flush.
As to 1, 2, or 3 drains. The fewer drains the simpler with fewer leak locations if you are an inexperienced plumber. The more drains the more redundancy in case 1 drain fails. No matter what I would do at least 2 drains for a Herbie style overflow. With 2 drains you can use one as a full siphon controlled by a gate valve and the other as a slow trickle drain. There are many videos on this drain style that explain it well. This method is far far far quieter than a single durso style drain and for this reason alone I would use at least 2 drains. In a 3 drain or bean animal set up the only difference is the third drain is an emergency overflow drain in case the first 2 drains clog.
I am personally not a fan of this style of drain as I think it adds more complexity with more potential leak areas. It is more reliability but my preferred method of increasing reliability is to just use bigger pipe. I have seen lots of 3/4 in drains clog a couple 1in clog but never a 1.5in or 2 in drain. My personal preference is to just go with bigger pipe but fewer lines if you are worried about something clogging. On your size tank most would use 1in drains. If you are concerned about clogging I would use 2, 1.5in drains.
