Tim: Though I have found MOSLEY to be one of the BEST tri-banders or multi-band beams around. I was always leery of the "All-in-One" package.
Those of us that live in the Eastern Mid-West (Ill/In/OH/Mich/KY) we can have every extreme of weather. Snow/Ice/Heat/Wind...Tornados/Straight line winds and even remnants of Hurricanes (as was the case for me IKE in the late 90's) where antennas can turn to piles of scrap metal in a matter of only minutes.
The "All-in-One antenna" in a fleeting moment can take you to great heights, to being OFF the air in one passing Spring/Summer storm.
My point is this, think of multiple antennas and not the all in one, where you may have lost one antenna but others are still operational.
My 2 cents IMHO
The used MOSLEY now: The difficulty in assembly would depend on "whom" took it apart.
Mosley always prided itself in assembly. Pre-drilled holes, excellent hardware, detailed instructions and color coded element pieces.
That being said, look the purchase over(in-person) before you hand out the cash and see how the disassembly was done.
Be prepared to contact Mosley for at least some "minor" hardware replacement parts.
Make sure EVERTHING is there. Look at the traps and look the hardware over...checklist etc...
Not distrusting the seller, however why are they selling...How long is it been down...How many times has it been "MOVED" since disassembly was done etc..
These all come into place when purchasing a used beam antenna.
OK personally i would rather have a nice tri-bander (I had a Classic 33 Mosley) and then another tower say with my 6 meter and 40 beam(
) or WARC bands beam on another tower. Good possibility one could be damaged other not.
I use wire beam on 40 meters( have good reports)(at home) but have used a 3 element 40 meter beam at the contest station (I operate from on occasion)
That antenna at 73 feet is a "killer".
Well just my thoughts, keep us up to date on possible progress.
Oh, another note: if that Mosley has the 40 meter kit with it, plus all the WARC bands basically 40 meters thru 20/17/15/12/10, I would say Rohn 45 minimum, most certainly if over 40 feet.
I personally would not use a Tailtwister on that big of beam(to small IMHO) and don't forget about 6 meters!!!!
All the Best
Gary/W9FNB