Looks like yet another frankenphone. Wish I knew where these things were coming from!
Its hard to tell you what to do, but if it currently had MD5 on there, you probably need to stick with T999 firmware. Well, maybe....
Install terminal emulator and enter this:
This will tell you what the firmware is. You cant judge by the rom or baseband.
Anyway, almost every time ive seen these things, the recommendation is usually to see if you can return the device. Theres just too many questions and things that dont match up. If you plan on using it as is, and never care about updates and such, I guess it might be ok, but otherwise theres just too great a chance for future problems!
For starters, there's no such thing as a GT-T999. And If download mode says its an I747, id guess youre using the I747 base firmware/bootloader. However, your system and baseband are from the T999 firmware, so we can be pretty confident that flashing I747 firmware will not work. But you probably can only flash I747 firmware because thats what your bootloader appears to be! Very confusing, very messed up, I know! Its highly possible, judging by the GT-T999, that it might be a chinese knock-off.
It may or may not help, but install Phone Info - Samsung app. Then let me know what it tells you. I dont think theres going to be a lot we can do though, but ill be glad to take a look anyway.