It's not lack of cash that's the issue I suspect - more and more I start to believe that the civilian government is more afraid of arming their own military than they are of their neighbours.
There are lots and lots of options out there that they could take up, really, yeah, BUY SOMETHING NOW!
Expanding a bit on your remarks...
The Argentine air-force has been budget-starved since the end of the Falklands/Malvinas conflict. The only substantial purchases where the A4-AR Fight Falcons during the 90's which, to my knowledge, are already at their operational limit. In contrast, the army did receive more equipment. The Argentine navy also did; plans for restarting the TR1700 submarine building program, or something of the sorts, and also minor upgrades to the P3B platform (2015) .
Receiving budget depended a lot on political alignment of the different forces with the ruling party, alas their air force was left to starve during the last government.
Things also get complicated as military expenditure is not seen keenly by some very loud crowds. Spending money in military upgrades now, would also heavily contradict the current government push to get the economy back in track (lowering expenses), with current inflation rates, alleged unemployment etc.. it would look like they are mocking their voters.
I wouldn't expect any new significant air platforms joining the air force's ranks this year, or even the next. I am pretty confident that there could be news in the navy aviations sector, but have yet to hear anything concrete in that regard.
Also, and this is fairly off topic, the fact that there isn't an arms race between South American nations hints that every single nation in it has their hands full with internal strife. Mercosur, OEA and other regional organizations failing to settle completely also points in that direction.