bhomburg

I would also check out Iberia plus and open an account with them in any case, even if you don't credit flights to it. Seeing you said you'll travel to South America, you'll likely end up on their flights as they have a large presence there.
IB uses the same avios currency as BA does, and you can transfer those between both programs, but need an account that's been open for a few months.
IB's program has far lower surcharges on award bookings compared to BA.

You'll only get lounge access as a mid-tier elite on oneworld ('sapphire' status). On airlines with mileage-based qualification, you typically have to fly 50.000 miles to attain it. BAEC uses a tier point system to determine status qualification which rewards travel in higher cabins. Type your most traveled routings into the calculator and see for yourself if you stand a chance of making Silver - should be possible if you have a few return flights to South America in Premium economy.