May an ESOP borrow money for any other purpose than the three uses (a) to acquire qualifying employer securities; (B) to repay the loan; or © to repay a prior exempt loan? For example, may an ESOP borrow money to invest in something other than qualifying employer securities?

I would think an ESOP could borrow for any purpose authorized in the terms of the trust agreement if it wasn't backed up by a party in interest or otherwise a prohibited transaction, though the IRS regs have things to say about the use of a loan's proceeds if the loan wants to qualify as an "exempt loan" (which allows for guaranty by the employer, etc.).

Any plan, including ESOPs, can borrow money to buy investments. But the earnings on such investments are taxable as debt-financed income (see IRC 514). Congress has this little problem with tax-exempts leveraging off their tax-free status!