You can rent an economy car, I have seen them many times but you really need to take it slow and watch out for rocks so you don't take off the oil pan. I have driven an economy car down there before. I will also mention this, there are times I've seen the road underwater and an economy car would not make it to Punta Allen. I was in a van and it made it down but saw an economy car stuck in the middle of the road bogged down from taking in water.
Yes you will need a boat and guide to reach the flats. Whether you pay 200-300+ plus is up to you. I have only fished with guides from the lodge I'm staying at. I have heard of people finding guides once they get to Punta Allen for less money but that it can be a crap shoot. You have to take some chances when doing it this way. I have heard of some horror stories from (my neighbor) and other postings on the internet. Problems like the guide didn't show up, showed up late, showed up hungover, came back early, no cold drinks onboard etc. I guess it can be one of those "you get what you pay for" deals. Good luck to you. Would like to hear how it went when you get back.
Have a good day,