My wife and I will be riding two classical Vespas from Cape Town to Nairobi this summer. This will be our first leg in our goal of cape to cape, and maybe some day an rtw.
In 2003 two guys rode from Hamburg to Cape Town on two classical Vespas. An Italian fellow, Giorgio Betinelli, did an RTW on a classic Vespa, and is now spending four years doing every single country in the world. Besides you and us, I know of atleast two other trips that are planned in the near future, riding scooters. Also, scooters have competed in the Dakar rally (in 1982 with four Vespa P200, no one placing, and in 1986 with two Vespa PX200 with one French fellow actually placing). So in short, you are not crazy.
As for mud and sand, you are wrong, your scooter with all your gear will have a tougher time than a motorcycle (smaller wheels, less power and less traction). On poor surfaces with wash boards, large bumps and pot holes, your scooter will be going much slower than a motorcycle. The small wheels and the short shock absorbers/springs simply can't cope. On semi good roads you will keep up with most motorcycles as fear of potholes, wild and domestic animals, etc, will slow them down to about the same speed as yourself. The really good roads, they will leave you far behind (but then, whats the fun in travelling fast on really good roads?).
Before I'd leave, I'd make sure I knew how to fix most things on my scooter. In Europe, North America, and much of south east asia, scooter mechanics are readily available. In Africa, the middle east and much of South America, finding someone to work on an automatic might not be as easy. If you don't have the means to repair stuff yourself (tools, parts and knowledge), be prepared to have to transport your scooter long distances on the back of a flat bed and wait for parts delivery Europe.
Get to know your scooter and bring along those critical parts which are most likely to fail or wear out quickly on your model, plus the tools to fix them. Parts and specialised tools can be difficult to come by in many places. Although you might not be able to fix it yourself, there is allways someone that comes along who can.
Worst case scenario is that you will have to have you and your scooter transported to the nearest metropol, order parts from Europe and wait for delivery (with your scooter, most parts should be readily available in European web shops such as http://www.sip-scootershop.com
You may think your scooter to be reliable, but it wasn't designed for such work. For instance, in Africa I will need to carry an extra 20 litres of fuel, plus an extra ten litres of water (under ideal conditions my scooter uses 3 litres/100km, but with the heavy loads it uses 5 litres on good roads and 8.5 on really poor roads). With all my extra gear, parts, tools, etc, the load becomes significant. Add to that an underdimensioned engine for theload and riding condtions, small wheels, underdimensioned shocks and clutch, poor road surfaces, extreme heat, steep climbs, long days, poor fuel, etc, and you've got yourself a ride prone to break down. Further, the scooter is not designed to carry any heavy loads. Even with serious modifications, your handling and performance will be greatly reduced.
You should ask yourself why you want to ride a scooter. A scooter is not a very practical choice. The only reason for choosing a scooter are emotional. You want to make some sort of a statement or acheivement by riding it, or the scooter has some sort of sentimental value to you. Now, if you are more or less only concerned about seeing the world from two wheels with wind blowing in your face, and you would prefer not to have to deal with all the hassles the scooter will give you, then choose something else.
Cost wise, you can get a brand new street legal 125cc offroad bike for the same price as the Gilera. Strengthen the sub frame, add some panniers and off you go. For the same costs still, you can get a used 350-650 cc off-roader (the ideal size in my opinion). But maybe you only hold a license for a 125cc?
In short, any bike and rider can make it to any place on the face of this earth. This is not the same as to say that each bike serves the purpose equally well. With some bikes, under some conditions, you may be carrying the bike rather than the other way arround.
My wife and I will be riding two identical Vespa PX200E EFL, modified with stronger clutches, saddle seats (to be able to bring rear top case more forward to distribute loads better), H4 lights, light grilles, new exhaust, stronger- and adjustable shock absorbers, specially made front and rear racks, luggage hoops, aluminum top case, 12v sigarette adapter, and much more. We will also, between the two of us, carry spare parts, tyres, etc, worth about 1.200 Euro + lots of tools (our scooters are not that reliable, and we can't afford any delays waiting for parts, etc). One scooter is today kitted with a 210 cylinder kit, which will be reversed to the original 200cc to increase fuel range and engine reliability. Cost wise, we could have chosen a far more practical bike. For me, this ride is as much about making a statement as experiencing the world from two wheels, as well as the nostalgia of classic scooters. For the trip I've got planned, in my opinion, riding a fully gadgetized KTM Enduro bike would be the equivalent of going duck hunting with an RPG7 rocket launcher. If you allready have one, great, if you don't, then use what ever fits your taste and wallet.
I'm very interested in hearing about what you ultimatly choose. Keep me posted.