DIY Chatroom is not a place to seek financial information regarding contractors' charges to do work. It is a how-to site. Seek estimates from local electricians to get the cost info.
Reversed polarity on the receptacles in the unpermitted basement finish would really raise a red flag with me if I were the buyer. Reversing the polarity of the receptacles means that someone has little or no understanding of what they were doing when they wired it or changed the devices out, and there's no way to say how bad they screwed up the stuff behind the walls. I'd make certain that you have your electrician look it over before signing the dotted line.
Running a circuit to the attic fan shouldn't be too much of a challenge for a competent electrician. The extension cords setup isn't a good thing.
Adding additional receptacles in the basement is a code-related issue, but may not necessarily be something that has to happen to make or break the sale.
Personally I'd be really hesitant to buy a property where the previous owner didn't care to have their work checked by professionals and things were done without permits. In my experience there is often a reason they don't want inspections.
Just to support his point, i had ungrounded receps in basement and they were wired with 14 gauge lamp cord, that brown stuff doubled together. I fixed everthing without a permit though. I follow code to the t when it comes to electrical. As far as i am concerned it was ther before+ i wiped the dates off the wire with acetone.