Well, getting ripped off is pretty standard with Porsches, so welcome to the club! ;)
Yes, a mechanic can tell if the rotors are bad by measuring them with a micrometer. If he had the car on a lift, he could get to the rotors easily without removing the wheels. He might even be able to do it with the car on the ground. To give you an idea of cost, I bought 4 brand new Zimmerman rotors (OEM) for $300. Of course a dealer or independent wrench will charge more than that, but that's what you can get them for.
As far as the motor mount, Modderman *might* have missed it. I can't say. Motor mounts do need to be replaced from time to time, and it involves jacking up the engine so that the mount can be replaced.
The clutch oil leak that he's talking about sounds like the Rear Main Seal. It is a very well-known problem with Boxsters. But it is not critical in the least - unless the leak is REALLY bad - which it probably isn't. Many people just live with a few drops of oil on their driveway and plan to replace the RMS when they replace their clutch. Something about this guy is not sounding on-the-level to me. I would pay your bill and get out of there.
Is this car still under warranty? If it is, don't spend another dime. All of this should be covered.
Did Modderman give you a report of some kind noting what they checked? Because if the motor mount is bad, the front rotors are bad, and you have an oil leak, it sounds like they didn't do their job, and you bought the car on their recommendation. You may have some legal recourse... I don't know. Again, I can't tell who's right or wrong in all of this, but I would start doing some investigating. First off, I would want to see the parts that he removed from your car. Always ask for your parts back - they're yours. And always ask BEFORE the shop starts doing work. This will let them know that you're smarter than the average Joe and will be keeping an eye on them. This is just being an educated consumer. If they give you a hard time about it, don't have any work done there. Take it to someone else.
If you can get your old parts back, especially the motor mount, take a picture and post it here. We can easily tell you if the mount is bad. As for the rotors, there is a minimum thickness that they should have in order to be replaced. I can't remember exactly, but I think a new rotor has 24mm thickness, and needs to be replaced when it has worn 2mm (down to 22mm thickness.) Front rotors do usually need to be replaced before rear rotors, as the car's weight shifts to the front when braking. How many miles are on this car? If you say 10K miles, I would suspect that this guy is taking you for an expensive ride. I got 90K miles out of my rotors before I replaced them. (This is not usual, but you should get at least 50K miles out of them.)
In the future, if someone tells you that something needs to be replaced, ask them how they arrived at that decision. Did he measure the rotors? Can he tell you the rotor thickness? What made him suspect the motor mount, etc.?
As I suggested, find another shop and get a second opinion. Then post back here with the second opinion before you spend any money. We will keep them honest and save you money. I hate getting ripped off, too. I'm guessing that 2 new rotors (from an independent shop) would cost $100-$150 each, plus a couple hours labor at ~$75 per hour. So that's $450 at the top end of the price scale. That leaves $1300 for the motor mount/labor and that is complete bullsh*t!
Take your car somewhere else.