Welcome to the Forum Juan...
Man your tall...LOL, therefore your Uncle is correct, a dualsport bike is going to fit you better than anything else out there. The KLR has the ability to be lowered or raised to fit the individual. You won't have to lower it. You are correct, they get great MPG, averaging 50 or better most of the time without exhuast or engine mods.
First, as you said, Do take the MSF course and listen to the instructors well and ask questions about things you don't understand or feel uncomfortable with. They should be able to help you with anything related to riding a motorcycle.
As for the square headlight, I like my square headlight..LMAO ok I am older and I like the older dirt bike style, but I do understand you seeing it as outdated. The reason for the square is because in 1987 when they first came out that was the design and it was good then. Kawasaki just never saw any need for improvement on the bike during its 20 year course.
Now for 2008 you see a much more street looking oriented bike with some nice design to it. It seems to be a love hate relationship. You either love the looks or you don't. It does have enough improvements over the older models to merit some serious consideration, but I have way too much time and modifications invested in my 07, that I won't be getting rid of it for quite some time, unless someone made me on heck of an offer for it.
Yes the KLR is one of the best selling dual sport bikes out. The amount of after market accessories is huge and many items for the 08 are becoming more and more available.
I can't think of any other brand that you might like better, but then I am kinda partial to my KLR
As for weather, if this is going to be your primary mode of transportation, you will need to fork up some bux for good riding gear that will 1) Protect you with pads and armour, and 2) will keep you dry. I have a bit of a handicap when it comes to being able to find good gear to fit, see I am wider than taller...LOL and there is very very little stuff out there available in my size, so I have to do the best I can. You will need a good full faced helmet, this keeps most of the wind out of your face and there are grip heaters to help keep your hands warm and also heated vests that can help keep your body warm and one manufacturer is making a seat that is heated also.
The best thing you can do, is just google KLR650 and start reading everything you can, also read stories of guys that have done trips to Mexico, Alaska, cross country and you will get a good idea of how dependable these machines are and also what kind of gear is available and how well it works. Sorry this has been so long, but hard to give you a short amount of information and all that you asked. Good luck in your search and decision.