Before buying and installing your AC, how are you venting you room? Do you have a activated carbon filter (Can Am or Phresh being most popular)? If you do not, save yourself some money (both now and monthly via electric) and install that. I was running a room that would peak up to 90/92 on hotter days, installed the proper size outake fan and can filter and stayed around 82 with lights on (not the ideal temp but also not too much above the ideal temp). You could vent that straight into your attic.
Your shower that vents to the attic have you been up in the attic? If your home is built to spec than there will be a 6"/8" flexible pipe venting to the ceiling and out your roof, if you do decide to install the AC and decide to vent to attic, buy a 'Y' attachment (may also need a reducer depending on size of hose you are using) and just piggy back off the existing vent system.