1. ZonaProp is good for BA, but what about the other 2 regions for searching ? Is it better to get an agent ?
The other regions you mentioned are tourist areas and almost all listing you'll see are meant to be for daily/weekend rentals (or up to a month) with very few actual long term rentals. An agent would make sense likely for these areas, but for Buenos Aires proper or GBA, I don't recommend it. Also, in the City of Buenos Aires you don't pay a realtor's commission, but in the province this can be passed along to the renter.
2. Is it possible to rent fully in Pesos and just pay directly via Western Union to landlord. Is this a typical arrangement ?
Yes it's possible to fully rent in pesos, but the paying directly via WU is not at all common.
Personally, I found my apartment on ZonaProp and proactively chose to offer to pay in dollars to my landlord for 2 reasons: 1, the dollar, blue/CCL/WU has not kept pace with inflation, meaning it is more expensive to pay in pesos than dollars, and 2, inflation is spiraling out of control and has resulted in the government permitting increases of up to 55% for last year in pesos (i.e if you rented a place for 40K ARS in 2021, your 2022 rent could be ~61K ARS), and god knows what it will be for 2023 with 90%+ inflation.
This is a way for both of you to hedge against inflation/devaluation, and some landlords given their wealth have accounts abroad making it possible to simply transfer it to their PayPal/Wise/ACH account, not having to deal with WU at all.