I'm looking to purchase a used car - I think I want either a 3 series BMW (probably a 318 or 325) or an Acura CL.

Whatever I get will most likely be a '97 or '98 and the cars I've found have 80,000-100,000 miles on them.

I won't get anything that's not in very good condition.

Which of the two would cost me less to insure?

And are BMWs really that expensive to repair? What about the Acura?

Is there anything you could tell me about either car that might influence my decision (good or bad)?

You can get instant quotes online from any of about a gazillion insurance companies. Geico, State Farm, you name it. It's a big decision, so do a little homework!

It really just depends on your driving record, and were you live.
Try the Progressive web site it's fast and easy to use. That's how I get quotes. http://affinity.progressive.com/entry.asp?url=www.progressive.com&code=8004009130&config=aumcbtrv
By the way be careful with the BMW. I have heard they have reliability problems.

the car has little significane to price, its more the record of the driver