C. 1159
There is already an arachnid robot. See here:
http://www.d20pfsrd.com/bestiary/mon...robot-arachnid.
Reduce size to Tiny if you want, though I don't really recommend it. Tiny creatures are just too weak because its claw attacks would be 1d2-1.
For the Small robot: Adding a Bite that does 1d2-1 points of damage probably has no effect on the CR. Adding the gun would increase CR to 1. This gives it the ability to shoot a bullet at 30 foot range. This would be the same as the type of firearm used (sized for a Small creature). Add siren (can't be stealthy w/siren). Siren would be like alarm spell as spell-like ability usable at least once a day and possibly 3 times a day if you want. It's not worth adding to CR.
Debby