I mean if I offer to pay (less) using USD bills instead of pesos with yearly 30% increments.
Many Argentine landlords don't let dollars affect them. They can be difficult to change for pesos and the market is uncertain to them - they earn in pesos and the government makes it difficult to deal with dollars. I recently signed a new 2 year lease, and out of some 20+ different apartments I looked at, only the landlords for my current apartment were willing to deal in dollars. One of the reasons I rented this apartment. Aside from being a really good deal.
The usual yearly inflation adjustment for contracts is 20-25%. I thought to use dollars as an inflation-suppressor, so that I didn't have to have the rate continue to go up in the contract. The best I could do was get the owners to agree to revisit which currency I would pay with in the second year, with pesos at an extra 20% as default. You may have luck or not - the range of owners' knowledge about the blue dollar and currency rates are quite varied.
Many Argentinos I know don't understand about the blue rate, which is one of the reasons they think it's complicated to do a contract in dollars and don't understand how doing it in dollars mitigates the need for an inflation expansion clause.
Getting a furnished long-term apartment is not going to be easy. They exist (at least I've seen some advertised as such), and many apartments come in various states of furnished anyway. My previous apartment, for example, had a table, a microwave and a washing machine that the owners left for us. Our current apartment, the owners were interested in renting out their deceased mother's apartment but 5 years after her death still hadn't emptied it before renting it to us. When we were moving in they offered to leave us a couple of couches, a dining room table and chairs, odd sitting chairs, a desk and a few other odds and ends, which we accepted, so they didn't have to store it or get rid of it.
But for the most part, long term leases are not furnished. One possible option is to go out to a "country" in the 'burbs and rent a vacation house by the year - they will usually be furnished. But much more expensive.
As for prices - the kind of properties you see that seem expensive related to local salaries, are upper-class places usually. As an example, I live on Quintana in Recoleta, a premium neighborhood for downtown. 170 sqm for $1,100 USD per month. Expenses about $2000 pesos, ABL something like $1000 pesos. All told, at the current blue dollar rate, about $1400 USD a month. It's an older building, no central air, although there is central heating. Some nice architecture inside as well as some crappy construction.
I've looked at places in Recoleta, Palermo, Belgrano, Saavedra, Nunez, Colegiales, etc. I found that the bigger places are all about the same price. Smaller places vary with neighborhood. The prices you're seeing on ArgenProp are correct and actually long term rental prices, at least for bigger properties, have come down quite a bit over the last 6 months or so.
You can go out to places away from the center or even in the province (i.e., outside of General Paz) and find houses and such much cheaper. Butyou have to deal with being out of the center, which without a car is not all that great unless you either don't have to leave your vicinity very often, or you like one or two hour long trips in trains and buses.
As for the guarantee - make sure that your partner's family, or whoever you all are thinking the guarantee will come from, are ready to sign their entire apartment over in case something goes drastically wrong with you guys. Please don't take this the wrong way, but some of your comments and responses seemed to show maybe that you have some idea of things here, but don't really have a good feel for the reality. The reason people use guarantees is because of the housing laws here - you basically can't get a Marshall or someone to evict someone from an apartment when they stop paying rent - an owner has to go through a lengthy court process, which could take two or more years, and still not be successful, to get squatters or non-payers out of their apartment. The guarantee is supplied to supposedly allow the damaged owner to receive the guaranteed property in return in case of severe damage or squatting. Many people will not even accept 6 or 12 or even 24 months of rent up front because of that fear - a bit irrational, in my opinion, when you are talking about someone with that much cash, but there it is anyway.
It's a screwed up situation, a great example where government gets involved to solve a problem and creates many more, worse problems. But understand that your partner's family will be asked to sign the contract, putting their property up for possible action. You may feel certain that you would never screw them - but they have seen people screw others over here like that and may be a bit more wary.