01 COBRA WITH CRACKED HEADS??

In a nutshell...

Not sure who told you our cylinder heads were prone to cracking, but it has not been a very common problem at all for the 99/01 engines. Run an engine compression/leakdown test to at least check the health of your engine if you suspect damage.

You're going to have to figure out where exactly that oil leak is coming from. The car could have been dogged to hell by its previous owner(s) and they neglected to fix the rear main seal. Have a reputable mustang shop look at it if you don't want to take it to a dealership.

The intake system will see a lot of oil deposited in it (a byproduct of Ford's PCV system). It is "acceptable" to see a puff of smoke (grey/blue-ish in color) at start up once in awhile due to the "puddled oil" in the upper intake. I believe the excessive amount of "puddled up oil" gets sucked into the combustion chamber and out the exhaust system. You can examine the puddled oil in your intake firsthand by removing the upper intake "hat."

Don't worry too much about it, unless your engine is consuming an abnormal amount of oil (check the oil level on a semi-regular basis - it's a good idea regardless).

As far as the rough idling, it could be a vacuum leak (check all hoses), dirty MAF, or a bad IAC. Check, clean, and/or replace the aforementioned parts as necessary.

G'luck troubleshooting!