BigDaveZJ
Adventurer
Build your own for sure. It's a great way to get back into welding!
There are so many +'s for building your own nd only a few for buying and modifying a military trailer...
The reason's I went custom:
Axle width matches my Jeep.
Bolt pattern, rims, rubber all match my Jeep (as spares if need be)
Electric brakes
Lighter
Better springs then a military trailer (CJ 2" lift front springs)
Now, you can do all that to a military trailer and end up with the same result, but the work to modify it is far greater then the work to build it to the same state. Mine is lighter then an M101, it has the Max Coupler, and I can easily add/remove parts as I see fit. Total cost is substantially less then an M101 (they go for $1000-$1200 around here quite easily).
I got 3 out of your 5 reasons for about half an hour worth of work. You still need to buy the axle for the homebrew trailer anyways, so there's no difference there.
If you want to spend a bunch of time cutting and fitting parts to build your own, go for it. I found it a lot easier to buy an M416 and do a few small tweaks to make it do what I want. I was originally going to build one myself, but considering the price I paid for my M416 ($350 in very good shape) it was certainly easier and less expensive for me to start with an existing trailer.