Dont worry too much on the mileage of the bike, anything under20,000 should be fine. Its more important to check the condition of the bike as a whole rather than just go by the clock.
You want something that has been cared for and requires nothing more than consumables to run nicely.
As you have just passed your test dont go out looking for too bif a machine. You'llbe on your own i guess for some of the time so whatever you choose has to be in your ability to pick up off the floor in case of any topples.
For where your planning on going your best bet would be a mid capacity 400-650 single cyclinder dual sport. Frugal on fuel, reliable and capable of good crusing speeds they can take a knock and dont stand out like gs's and bigger tourers.
But get some practice in on any bike first