You know you want the XT....
I may be a little biased cos I've got one but honestly you can't go wrong with an XT600E.
It will do almost anything well. The only thing it struggles with is high speed motorway work. Providing you are happy "poodling" around at no more than 70 mph, It's not a real problem.
A truly bullet-Proof engine, good build quality and plenty of power. It handles well too around town. No need to go into pedigree with regard to RTW trips, it's been just about everywhere on earth and is probably the No1 adventure bike when it comes to Sub-Saharan African trips (followed closely by Beemers).
I procured my XT in January 2008 for only £1,600. It's in superb condition and has only 5,000 (genuine) miles on the clock. There are still plenty of "goodans" about. No need to pay more than about £1,800 for a late XT600E and £2,200 for a nice XT660 (It's direct replacement).
By the way, how old is too old? I'm of 1963 vintage (44) and still capable of a few miles yet...
Oh yes, Welcome to the forum...
Triumph Bonneville 800 (2004), Yamaha XT600E (1999), Honda XBR500 (1986).