Given the requirements needed from said machine... ie.  cheap to buy, solid build, very reliable, spares plentiful, very economical, powerful enough to do what will be asked of it, easy to maintain during travels, comfortable, good luggage capacity....   i've probably missed something, but decided on the old Honda CB500, as it fitted all these requirements (& my last one proved it's worth, taking me from Derby to Dakar/Senegal, Africa)