I suggest you contact patriciafromuruguay (a member of this site). She helped my get my residency and DNI and has helped several other expats from this site. I don't know how much lawyers charge to help with residency, but she charges $200 per person to go to migraciones with you (to act as a translator). She was able to answer questions and overcome objections I could not have dealt with. She will not charge extra if you have to return to migracions for any reason.
The most important issue you will face is if you have enough income from your Social Security to qualify for the visa pensionado.
Just multiply the amount you receive in dollars by the official exchange rate.
The last rate I heard for the visa rentista was $8800 pesos per month.
The income requirement for the visa pensionado used to be unspecified and a lower level than for the visa pensionado was considered acceptable.
Migraciones may be looking for the same income level now for both types of residency.
If you can combine your Social Security incomes when you apply you will certainly exceed the current amount and
you will be granted temporary residency.
On the third renewal you can apply for permanent residency.
Or you can apply for citizenship, but that will require paying a lawyer much, much more (since you don't already have the DNIs).
I recently received a PM from a member of the forum who indicated the latest rate was $6500 USD per person.
There really isn't anything a "lawyer" can do for you at migraciones that Patricia can't...except charge you much, much more.
You have to order your own documents and that's not something an Argentine lawyer can do for you. The FBI report requires the most effort and takes the longest amount of time to get. You must also (easily) get the Argentine police report yourselves.
The Social Security Office at the US Embassy in BA can certify you income and you can have that document legalized in BA.
You can make the required photo copies of your passport at any place that makes photocopias. You can get the required photos at any Kodak kiosko, and finding a translator for all of the documents is easy because all of the ones who translate document from English to Spanish speak English. There are probably several within walking distance of your apartment.
PS: I don't think the fact that you own an apartment in BA makes any difference in being approved for temporary residency.
I will send you Patricia's email.
Nice to know you are finally coming to Argentina to live.