A classic tourer would be a Dawes Galaxy. At the cheaper end, you should also look into older steel mtb's with a rigid fork. The better ones had nice frames, and can be had very cheaply. Fit a good rear rack like a Tubus Cargo, avoid taking so much that you need a front rack (easily done), fit wide slick tyres like 50mm Schwalbe Big Apples, and you should be all set. Of course, as with all second hand bikes you may need to replace wearing parts like cassette and chain, brake blocks (Koolstop Salmon are the best) and for your safety the brake cables.
Willem