Well, Sven, I'm glad you asked.
You can get stationed over in Japan and be a vehicle mechanic. But you have to make sure that stuff is written in your contract. And they'll only draw up your contract after you go through most of the administrative hoops first. Which reminds me- did you take the asvab or the practice one they have on the recruiter's computers? Cuz you'd have to take the real, official test at the meps station on post. And generally you do your medical evaluations during the rest of the day at the same building. Then they tell you what's available for you and generate your contract.
Read through your contract and make sure your chosen job is stated in there, and that you will be stationed in Japan after completion of your training. If you do go through with it, they may offer you bonuses to enlist for longer. I would highly advise sticking with the basic 3 or 4 year contract. That way you can decide whether or not to stay in, and if you re-enlist, it's like signing the initial contract again and you can get it in writing to re-enlist and stay in the same place, or go somewhere else you want to go. This way gives you choice, and you want maximum choice because after you sign, the military will make a lot of choices for you and you will dislike the vast majority of them.
Whether enlisting or re-enlisting, they may try to low-ball you. Know exactly what you want and don't settle for anything else. They've got different quotas for different jobs as well as for overall enlistments so a lot of guys will tell you the wrong things and try to convince you that something isn't available right now and that this or that job could be fun for you too. Don't buy it. Just leave and tell 'em to call you when it becomes available. Slots open and close, and update every month if not every couple of weeks. If they suddenly double check and find that it just updated and they found an open slot you wanted... well, triple check your contract.
Reserves and national guard DO get deployed and pretty often. A reserve unit of MPs was deployed in the major city in our area of Afghan while we mostly covered all the smaller towns and borders. But with the drawdown and all that, the likelihood of that is small and growing even smaller. And in any case, mechanics are pretty well protected even if they go to the front line little places where they stay and maintain the infantry's vehicles.
As for going to basic training then coming back for college- the reserves and national guard are totally legit with that shit, but to be sure you'd best make sure it's in your contract. And even if something crazy were to go on overseas or nationally, if you're in the program they can't deploy you or send you anywhere else until after you get your degree. When I was in the guard, it was split up to do basic training in the summer so you don't miss semesters, and then if your particular job training won't take too long, you'll also go through training for that during that summer too. If it would be too long and interfere with the starting dates for the next fall semester, you go to school as the priority and go to your job training during the next summer. No school missed.
Also, do you have college credits already or will have some by the time you figure you might join? That and some other things can determine what rank you will start off as when you join. If you have so many credits, you can start as an E-3 or E-2. I think you'd have to have a degree already to go in as an e4, so yeah, that's probably right out of the question. If you don't have any, there's still some other stuff you can do to enter into the army as a higher rank than the super basic E-1. (You'd have to ask 'em specifically what you'd have to do, but I'm pretty sure passing the full pt test before going to basic training is one way, and there are some training things through the recruiting station you can do and you'll get promoted for getting that stuff done). The higher you are when you start, the better. Mostly for pay reasons, of course. You'll all go through the same basic training and do the same shit, but they'll get paid more.
And it's not really like I joined the army to serve my country. And you can grow longish hair back later. In basic training, you will be skin-blade bald.