What you want is a brand new Japanese bike, preferably Honda or Yamaha. Honda is the most sold brand accross the globe, also in the third world, with Yamaha as a close second. This translates into availability of parts and technical know how accross the globe, and lots of after market parts for the most sold models. Most Hondas and Yamahas have excellent reliability. There may be small issues with different models, as with all bikes, but usually things which can easily be resolved by fitting a superior after market upgrade for the particular part. In general though, they have few problems.

If you find your 650 a bit too big, then you don't want anything bigger than 450-500 cc. On the other hand, performance wise, you don't want anything smaller than 250 cc. For your intents and purposes, it sounds to me as the ideal is somewhere in between 350-450 cc. I must underscore though that the engine displacement is not allways positively correlated to the bike's weight, but it is often a good indication, and a good rule of thumb.

Ideally the type of bike you want, is a Honda or Yamaha Enduro/Offroad hybrid bike, of 350-450 cc with a strong sub frame and a comfortable seat. The only problem is that this bike does not excist. Your best bet is then to either live with the flaws of the available bikes, or do a bit of modification to whatever bike fancies you (seat, subframes, shocks, etc.). With a brand new bike, this will cost you a lot of money which you will never recoup if you were to sell. In fact, the money you sunk into it may actually devalue the bike.

As there is a long time until you will be riding, and you are interested enough to pursue technical classes, my suggestion is to buy an older Japanese offroader or motorcross bike. Join a forum for your bike, buy some tools and manuals, and take the entire thing appart and put it back together, every nut and bolt. In the process, swap all worn parts, strengthen the frame and subframe, fit the after market upgrades you want, and so on. Even if you have never done this before, you will be able to do it... if you've got patience, time and stamina... no prior experience needed, promise.

You will never again need a mechanic, it will make you even more passionate about your ride, you can build yourself the perfect ride, and you will save money in the long run (not in the short as tools and parts are going to cost you). You will of course become frustrated as things move painstakingly slow, usually in the form of waiting. Sometimes you do only 10 minutes of wrenching before you feel too uncomfortable to move on without a piece of advice. You place a post and have to wait for the replies. Then you wrench another few minutes before discovering that you lack a particular tool. You run out to buy it. You do some more wrenching, only to discover you lack a particular part. Your supplier is out of stock, so you have to wait. When you receive it, a week too late, you discover that it was the wrong part. When you finally receive the correct part, and have the new part fitted, a new problem occurs, and the entire process repeats itself, over and over and over. But don't worry, you quickly get much better and faster at it. In other words, if you pursue this route, don't sell your other bike before you've got your new custom overlander on the road.

Good candidates for you would be Yamaha's XT series or Honda's XL series. A bike less than 10 years old, and a bike with less than 50k km on it, and one which is in original condition and looking good. There are three reasons for choosing a bike with low milage and age. One is that the metal in the frame will not be as tired, the second reason is that parts are more easily obtainable for newer models than older ones, and the most obvious reason is that you will not need to sink as much money into replacing worn parts. But, if you are to take it appart anyways, every nut and bolt, and you are prepared to strengthen the old frame, etc, then you can go for much older bikes with far more kms. If you are real brave, then you could even try to customise a touring bike into an overlander...

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