First word of advice is contact the place that will be printing your ads. If it an inhouse thing then you shouldn't have to worry about overprint with CMYK.

I personally turn off all compression when I am exporting ot PDF for the stuff I do. Most of it is line art type stuff so the file sizes don't get too big. The reason I shut it off is because I don't want to worry about anything getting down sampled and causing it to look bad when printed.

Honestly a lot of thise depends on where you are printing it.

Moonowl,
I'm with Neuro on advice: pick up your phone and call them and ask them just what you asked in this forum. I know that sometimes designers don't want to ask the printer they want to appear as though they already know what they are doing, etc., to sort of save face and look professional, but trust me there is nothing unprofessional about going in to the printer with your heart on your sleeve and saying "School me up Please! Tell me what I should and should not do to have my files perfect when I bring them to you" the printer will tell you, it's that easy. And you only have to do this once and then you know what he wants and he won't have to call you if something isn't right, etc. I have earned alot of respect from printers by doing this very thing. If I'm not sure I call and ask, the graphics people at print shops usually appreciate you trying to find out how to do it right. It saves them time later when your job comes across their desk because they won't have to stop and fix things. It never hurts to ask questions. One thing my best and favorite printer always likes is the file in pdf and in InDesign just incase something needs to be tweeked. Good Luck!

Always ask, they may just take INDD files. OF a PDF X1-A file should do. As for your overprinting of elements ....that's up to you. If you aren't well versed in what happens when inks are overprinted, you might not want to start until you are. Don't practice on jobs that are going to print.

Overprint dark colored type? Unless you are looking to acheive a artsy effect that dark colored type should be black. You'll need to tell the printer what build that dark colored type is. A good printer will decide to overprint it or make it a rich black (4/c process job) or knock it out of the background.