There is a lot of information about these bikes on this site.
MT Riders Club Forum
The 350 is a very sturdy bike, well built, reliable when maintained properly and a very capable luggage carrier. The 350 is a bit underpowered but does come with disk brakes and electric start (as well as a kickstart)
The 500 Armstrong is an older model, with drum brakes and no electric start, but otherwise very similiar.
Armstrong were supplying the 500 to the army when Harley took over the manufacture and they continued with basically the same design.
The Rotax engines are virtually bullet proof and were also used in CCMs, Aprillia, MZs to name a few. They have the advantage that all the Rotax engines in the range, 350,500.560.604,640 have the same physical dimensions and are interchangeable, with one drawback that an electric start can't be fitted to a 500 frame without a bit of cutting and welding to the front frame member.
It's now a popular upgrade to fit a larger engine to the 350 chassis.
The bikes are heavy for any serious off-road work, but they are ideally suited to dual-purpose use and green-laning and certainly no heavier than many of the more popular adventure style bikes.
Withams are well known ex-army surplus auctioneers who sell these bikes without any preparation from £400 upwards. You may be lucky or you may get one which has parts missing and needs a full rebuild. Force motorcycles in the midlands, Ivy Leigh in Liverpool and LMS also from the midlands will sell them reconditioned with a warranty, but they are nearer £2000.
Privately the good ones go for around £1500.
I would happily set off on an extended trip with mine. In fact that's why I bought it. I've had it now for nearly 2 years and I'm currently in the process of fitting a 640 engine to it.
It's got no chrome on it to go rusty, no exposed aluminum to corrode, no shiny painwork to go dull. It doesn't get washed or cleaned for months. It looks scruffy and cheap so doesn't stand out, but it's mechanically sound.

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