How accurate does the handling of the cars in the game compare to the real world?
Honest truth? Right now it's low. Most of the cars still have placeholder physics. The tyre model is very complex, and is still having features added and bugs worked out of it. See the videos of the flipping Formula A. That one is being squashed as we speak.
On the other hand, a few cars that have been worked on for a substantial amount of time are supposedly very good (although they're still designated as not having finalised physics, due to the ever changing tyre model). They feel good to me, but I wouldn't know about accuracy because I'm not a race car driver.
However, Ben Collins (ex-Stig) and Nicolas Hamilton (brother of Lewis and a race driver himself) have been giving feedback and are fairly positive. There are other people with racing and track experience in the project as well including professional race drivers (sorry I can't remember names off the top of my head), and they HAVE driven these cars on some of the tracks in real life. SMS also have a lot of data from manufacturers.
Basically, at the moment it's a little bit of very good, a lot of kind of OK, and some horribly broken. What do you expect from a game in progress?

But it can be seen that the capability is there to make the cars very good and very accurate, and the manpower is there in the community to hopefully catch any mistakes before they go out.
I expect by the time the game is released the cars will be as accurate to the real thing as they can make them. There's substantial effort going into each car to make them just right.
iRacing is probably the gold standard for car accuracy. Whatever you think about their physics, they put a ton of time into each car. I expect pCARS to be in-between that and GT, and I hope that they can stick it to iRacing. If they get the tyre model right they well could, because even two years after NTM 2.0 iRacing still has serious issues with tyres.
While its true that the funding will only open for the next gen versions (as in: you will only get profits from sales there) investing in those versions will probably still grant you access to the PC builds AFAIK.
Correct, it will grant you access to PC builds.
I'm merely being coy about the potential of next-gen being available as a funding option. I know that at the moment the intention is to offer it eventually, but I don't want to state it as a certainty because the plans may change.
IF next-gen funding is offered anyone who takes it up will get access to the PC builds. Sorry if that was unclear.
The truth is that while all of us who are in the project now are funding PC/X360/PS3/WiiU, we only have access to PC builds.