If you have any mechanical skills you should be fine doing it yourself.
Goldline makes a PS-4. It is a SWG and automation. If you go with this one be aware that you have to purchase the actual unit, remote sensor and remote. Several different remote options, wired, wireless, handheld wireless etc. You can even run your landscaping lights if you want.
Keep in mind you'll need special valves to use automation. Either Jandy, intermatic, etc. The ones that have a back or grey handle on them that can be removed. Since you are only changing pool to spa. You should only need 2 actuators. Depending upon how your system is plumbed.
Like Jason said the cost varies greatly depending upon the "bells and whistles"