I'm a SPC in 11B stationed at FT Hood, TX. I ETS from my first active duty 3yr enlistment near Mar08. I was prior service 6yr in National Guard. I've had 2 overseas tours 6mo+ in duration each in a combat theater. I got a lower back injury prior to my last deployment and it eventually caused me to be MedEvac from theater and have spinal fusion. My doctor feels that I am well enough to begin MMRB or reclass. I want to go into a technical MOS in an electrical/mechanical repairer field because that's what I'm good at and enjoy. All of my test scores are above 115 (ASVAB dated around 1998 is 78pts).
The MOS I would prefer is not listed as a critical skill. It is MOS 68A Medical Equipment Repairer. But there are several other MOSs listed that I would like to go for. Where do I find the qualifications for those MOSs and strengths. What are my options that my Career Counselor may/may not tell me about? Can I expect a bonus for a critical skill MOS/how much?

If you're going for an MMRB, you must wait until the results are returned and you'll be given about 4 MOSs to choose from; that you are medically qualified for.

Just because the MOS is not listed on the critical MOS list does not mean you cannot get that MOS.
Read the Medical Reclassification procedures:
http://www.armyreenlistment.com/Messages/Reclass/Reclass_07_06.pdf

For qualification of each MOS, go to the below link and click on MOS Book "Download":
http://www.armyreenlistment.com/careercounselor.htm

About your options your Career Counselor may/may not tell you about ... what makes you think your Career Counselor may not tell you about an option?

You cannot expect a bonus for your reclassification because you are not yet qualified in that MOS.