My question is: if I file a provisional patent application in the USA, what's stopping someone from scooping it up and going international before I'm able to file a PCT? If so, in the first 12 months should I avoid doing international business? Or, should I skip the US patent altogether, knowing our market is international?
A prov is not a traditional patent Application, it is meant to file a disclosure and lock in a priority date. The provisional application is not published unless you claim priority from the provisional. If you do claim priority, the provisional application becomes open to public inspection (but will not have a publication number like patent applications and issued patents have), and all applications claiming priority from this provisional application are published 18 months after the filing date (that is, the priority date) of the provisional.
Filing a PCT is NOT an "upgrade". It is filing of an Application, albeit one where a patent is not granted, and cannot be enforced. To get a patent following a PCT application, you then have to enter National Phase in each country you want a patent/protection in, which has to be done within 30 months from the priority date for most countries, 24 for a handful.
A PCT (or a patent application in any country) has to be filed within 12 months of the priority date (filing of provisional), beyond which you lose the right to that priority date. That is, your filing date becomes the priority date of the PCT or any other patent application if you miss the opportunity to file within the 12 month window after filing the provisional.