Any bike can be used for touring long distances. The main question is, what type of touring do you want to do?
If you want to do self-supported touring (you carry the luggage yourself on the bike vs. a car transports the stuff for the whole group), you need a bike that can take luggage. The other main feature I look for in a touring bike is comfort, because I spend a lot of time on the bike, 3-5 hours a day, day after day.
Most mountainbikes, citybikes, trekking bikes, comfort bikes can be retrofitted with racks/carriers to hold panniers. Usually the rear-carrier is easy to do, the front carrier might be tricky, but there are front carriers even for suspension forks (see Old Man Mountain front racks).
For me, the most important feature of a specifically long-distance touring bike is the frame geometry. And in frame geometry, for me in a touring bike, the most important is the chainstay. A long-distance touring bike has a minimum of 45 cm chainstay (vs. 40-41 cm), but touring specific manufactures do longer ones (Rose bikes: 46 cm, IDworx: 47.1 cm). This is to give space for rear panniers, and usually the same for all frame sizes. If your feet hit the rear panniers, you cannot go. I had this problem, and it really is a problem, and difficult to fix. After checking some bikes, you will be able to tell visually how long the chainstay is (big distance between rear wheels and seatpost). To be very precise, you can measure the actual chainstay (the horizontal distance).
WHEELBASE Another helpful feature is a long wheelbase, which is size dependent, but usually 105 cm and up. This is helpful for the slower, in line motion of touring (vs. fast turning in a group of bikes in a competition).
The rest, you can upgrade, or live with. My wife and I, we tour a lot. She has a 7 speed (hub gear) city bike with a rubbish rear roller brake (not to confuse with a disk brake). I have a 8 speed (hub gear) city bike-mountain bike hybrid. We bought some panniers, and there we go. On steep hills, we get off and push the bikes. I did 3000 km in the last year, and we've just been in Slovenia on a few hundred KM tour, last year we've been at the Dolomiti in Italy. We complained a lot about the 7 and 8 gears, but it didn't stop us from touring.
Some myths that I think you should NOT consider:
- "Steel frame is easy to fix when broken". These days all frames are of good quality. If your frame is broken, you will order anyway a new frame, because a welded-repaired steel frame is no use at all. I have one bike with a steel frame, after an accident the frame got bent, and the bike doesn't really keep a straight line. It's not possible to get it back to the original condition.
- "Easy of repair" in general. If you tour in Europe, for a few days, you will always find a repair shop, and if you need special parts, you can always order by mail in 1-2 days. Bar end shifters are easy to repair, and this was a must 20-30 years ago, today is less of a concern. If your wheel is destroyed, or your tyre falls apart, you will order anyway a quality wheel/tyre, and not fiddle with a third world low quality tyre. If you look around the web, people do around the World tours with disc brakes, carbon drive, and other fancy stuff, and if they break down in the middle of Africa, they get the parts by mail order in 1-2 days, and continue.
- Kona Sutra, Surely LHT, Trek 520 are the best Many websites are written in English, and are maintained by US, UK tourers, and list brands from these countries. However, bike touring in The Netherlands, and in Germany is a much bigger thing then in any other country. They have lots of brands, even premium ones, only for touring. See for example IDWorx, Koga in the Netherlands, or Rose Bikes, Tout Terrain in Germany. They have a long list of different touring bikes. In Amsterdam there is a bike shop only for touring bikes (De Vakantiefietser). Only touring bikes!
- Some very special bike is needed. When I started, I talked and asked a lot about the bikes. The more I go on touring, the less I care about the bike, and focus on the trip.
By my experience, the most helpful thing about the bike on a bike tour are puncture proof tyres, because it saves a lot of time for relatively smaller investment. Schwalbe Marathon tyres are like that. Any bike you buy for touring, I recommend to change the tyres to Schwalbe Marathon (or any other Schwalbe that is puncture proof), this will save you a lot of time when on a trip.
I suggest to buy the widest tyres your bike can take, for added comfort.
In summary: my recommendation is
- to check for a 45 cm (or more) chainstay to fit panniers easily.
- Schwalbe Marathon tyres to avoid punctures.
- The rest, you can change, upgrade, make the bike comfortable for yourself within the your budget.
You want to read more on here:
Touring bike manufactures, grouped by country: Cycling About
What to look for in a touring bike frame: Cycling About