I don't have many SCSI II drives, but if you are looking for 68 pin, I have a ton of those drives ranging from 4GB all the way up to 36.7 I believe. 5 or 6 of them are Cheetah drives I think. Good luck with getting the drive you are looking for!
I don't know about the adapter thing. I do know that 50-68 pin works great, but I don't know about the external 25->50->68 as I use mine inside my A4000D. It still requires a molex power connector though. The biggest Drive I have is am IBM Ultrastar, which is 36.7GB. I actually have two of these by the way. Personally I think it is a little loud, so if you want one that is quieter, I have a Cheetah Ultra 320 this 34.1 GB this is actually a little quieter. If quiet is your main concern I till plug a bunch of drives in and listen for which one is the most silent if you are curious.