Hi and welcome to our site!A) If my aunt who is not a US citizen were to give me $100 to $300k would it be taxable to me in the US?The gift is NEVER taxable income for recipients. Gift itself - is not taxable income in the US. Please see for reference IRS publication 525 page 31 left column - - http://www.irs.gov/pub/irs-pdf/p525.pdf
Gifts and inheritances. In most cases, property you receive as a gift, bequest, or inheritance is not included in your income. However, if property you receive this way later produces income such as interest, dividends, or rents, that income is taxable to you.So you will not need to claim that gift on your tax return. There is no any amount limit. That is for income tax purposes.
If you are US person and will receive a gift from nonresident alien - there is additional reporting requirements. The fact of receiving the gift from a nonresident alien is separately reported on form 3520. -http://www.irs.gov/pub/irs-pdf/f3520.pdf. In general, Form 3520 is due on the date that your income tax return is due, including extensions. There is no tax associated with that form. Only gifts above $100,000 are reported.
In addition my wife lent my fathers company in Mexico $100k. B)Would she be taxed on getting that money back? When the person takes a loan - it is not taxable - because - that amount is expected to be paid back. Correspondingly - a creditor may not deduct the amount he/she loaned to someone. So when the money are paid back - that is not a taxable income. However interest paid on the loan is taxable for the creditor. If there is no interest - the IRS uses so -called imputed interest based on published Applicable Federal Rates (AFR). That that interest would be taxable.Rates are published here - http://apps.irs.gov/app/picklist/list/federalRates.html
C) Do you know a Mexican tax attorney who would know if there would be withholding taxes in Mexico since the company didn't document my wife's loan. She wrote checks to the US bank account for the company in the companies name.There should not be tax withholding form the loan. It is commonly accepted that loans are not taxable income. In additional both Mexico and the US are members of a free-trade agreement - NAFTA - and there should not be any additional taxes on money transfers between countries. Unfortunately I may not recommend any Mexican tax attorney. But I am sure that lots on Mexicans are sending money from the US and do not have any issues. Still I might suggest to have a formal loan agreement. When the check is deposited into Mexican bank - before - verify if the bank changes any fees and you may compare fees from different banks.