Povo,
It's not really that difficult to install an air conditioner as long as the vent is cut in the roof and the wiring is installed next to the vent. The biggest "hurdle" is getting the unit onto the roof. After that, it's really not difficult at all.
Here is a link to the installation instructions. Look them over and see if you believe it is within your "level of experience" and your "physical capacity" if it is, you can save yourself $300 to $500 bucks with a "do-it-yourself" job.
http://www2.dometic.com/1f24ead8-631...87ce6bad.fodoc
Usually, for someone who has never installed an air conditioner, the time from start to finish (actual work, not chilling beverages for after the job) is about 2 hours.
Good Luck