I used one for a 2 month tour of west europe back in 1987. Did about 8000km and loved it. Rear shock seal blew but other than that it ran realy well, just had to rev it more than my mate on an XT600. Went with a smaller rear sprocket on the rear and a choice of 2 for the front ( motorway cruise and dirt). Only spent a few days at 120kmh on the big roads but it handled it without problems.
Also seen a couple used as RTW tourers, they work well (Iran and Mali).
And now the opinion... would much prefer the 400 for playing in sand and the dommie for roads. The 250 I would prefer for the general touristy stuff and fun of it. Sounds like a bargain with the ex army one, that would be for me, sadle up and go..