I picked up the second hand LSD E153 box from Gorilla last night, and dropped it at Gearbox Express this morning so they can inspect it. 
Having just talked to Gearbox Express after they pulled the box apart, here is the latest news:

- The synchros for 2nd and 3rd gear aren't looking good

- The little gear on the electronic speedo sender is broken, but I guess that doesn't matter because I'd be retaining my mechanical speedo anyway

- There is no 'hex bar' visible when looking into the stub axle holes in the gearbox, which should imply that it doesn't have a factory LSD, but Gearbox Express are convinced that it has some sort of LSD

Regardless of the LSD, the pricing is another major factor.
They said it will cost $250 if they simply replace some seals and put it back together as-is, which is a reasonable amount.
However, if they were to fix the synchros before reassembling the box, it would cost a total of $1400 - ouch!
Also, both of the above options are not including the cost of the box itself from Gorilla Industries.

I can still get my original gearbox repaired if I want to, but it wouldn't have the benefits of an LSD or revised synchros.

So much to think about! lol

Fix current gearbox +  Quaife ATB unit 
problem solved :P

Given the cost of fixing the synchros in the second hand box, I am certainly considering that idea.
I could look for another second hand box of course, but it wouldn't be worth the hassle when it could have the same issues or worse.

Any findings on what failed on the original box?

My car is still in the garage and the original gearbox hasn't been removed yet for a proper inspection of the problem. My mechanic will be doing that, but the car will need to be towed to him, and I don't want to leave the car with him for longer than necessary.
I still think it's very likely that the 5th gear shift fork has broken, and possibly damaged some part of the gearbox casing, and therefore the fluid leak is probably gearbox oil.
I guess it would probably cost a lot less to fix the shift fork on my existing box rather than fixing the synchros on the other box, but I still have to consider the LSD situation (after confirming what type of LSD the other box has, if it has one at all).

When looking through the axle hole what do you see?
So no hex shaped bar, does it look like this pic?http://i235.photobucket.com/albums/ee93/lamda747/Image00007.jpg
If so I'm almost 100% sure its a TRD lsd unit.

Bummer to hear that... I'd put the E153 box back together and not worry bout it.  That price is ridonkulous - $1400 for new synchros + buying price + not knowing whether it has an LSD or not. 
Then pull out your old box and it might be cheaper going down route MRturbo suggested - Quaife unit with your old gearbox.  Guess it depends on how bad the damage is...

Mrskylighter
When looking through the axle hole what do you see?
So no hex shaped bar, does it look like this pic?http://i235.photobucket.com/albums/ee93/lamda747/Image00007.jpg
If so I'm almost 100% sure its a TRD lsd unit.

I should have taken a photo looking into the axle holes last night, I forgot. There is definitely no hex bar. I can't remember if it looks like the above pic, ie. TRD LSD. I suppose it could be.
Was the TRD LSD a factory option, or would the box still have been opened at a later date to have a TRD LSD fitted?
Is the TRD LSD viscous like the factory one?

Its a pitty you didn't look for the hex bar before you dropped it off at the gearbox shop.
Does this new box have the upgraded synchro design in it? That would prove its at least a late model E153 box.

Do it yourself ;p
http://www.mr2oc.com/showthread.php?t=393364

LSD?
http://www.mr2oc.com/showthread.php?t=447263