I've been offered a 125G acrylic USED tank with some LR.

My question is what signs should I be looking for? I'm afraid of leaks or
parasites on the rock.

Fill the tank with regular tap water and see if it holds water without
leaking for a day or so, then you will know if it's ok. As far as the LR,
find out if its been sitting around for awhile or has it been in a tank. If
its in a tank, I wouldn't worry too much about it. If its been sitting, you
may have to cure it yourself. Seeing how this will be a fresh tank, you can
probably cure it inside the tank you're getting.
How strong the stand has to be to support such weight?

Go to your local LFS and see if they have a tank similar to what you're
getting, if they do, then you can see what type of stand you will need, if
they don't, Acrylic weighs less then glass; water is roughly 8 lbs per
gallon. Find something you like or there are plans on the net for a DIY

Also, if I get it, metal halides are a must, but, I heard the should
be
keep inside a canopy for safety reasons, how true is that?

Metal halides are a must for certain types of corals. If you're running a
FOWLR, then you wont need it