The Transalp is an excellent choice. Make sure that the engine turns over easy (starts easy).
All types of bikes, new or old, will get you where you want to go, but that is not the same to say as all are ideal.
In general, I'd go for a one cylindered bike of less than 650 cc, less than 10 years old, and with less than 50.000 kms on the axle (ideally less than 30k). Some will tell you that older bikes will be just as good, but in general parts will be more readily accessible to newer models. Low kms is not a guarantee of a good condition, but it is a benefit. These bikes often need major work at 100k. If it looks great and only has a few k, this is often a good indication.
Other bikes to concider in the same price range as the transalp:
Honda Africa Twin (a bit big, but still good)
Yamaha XTZ Tenere (this is a great bike, the king og its class during the 90's). The later Super Tenere is a bit big, but still a good bike.
Even cheaper bikes (but usually a little bit older): Honda XL, Yamaha XT
Service the bike before you go.