I wonder if I need to purchase a return ticket to be eligible to enter the country?

You should apply for a Argentina visa with the following documents:
a. Photocopy of two-way air ticket.
b. Proof of financial ability to meet travel expenses (such as: savings or bank account under own name, or last submission of tax return, or any other documentary evidence indicating enough solvency to withstand average expenses of US$ 150.00 a day for the time duration of the intended trip.
c. Photocopy of biodata pages of travel document.
d. Photocopy of Hong Kong Identity Card.
e. One recent passport photograph
f. Consular Fee is HK$ 233.70