1. Can only do the job with the same occupation that is specified on the work permit.
This is an open work permit so you can work in any field but only in ONTARIO . But there are some restrictions as written on Work Permit Paper :
You Can not Attend any educational institute and take any academic , professional or vocational training course. ( Which means if you want to do this then you should have Study Permit not a Work Permit. ) But if your kids have Study permit they are allowed these facilities.
Not authorized to work in Following Fields:
Child Care, Primary or Secondary School Teaching, Health Services.
It also says :
Not valid for employment in businesses related to SEX TRADE such as STRIP CLUBS, MASSAGE PARLOURS or ESCORT SERVICES.
2. Can not apply health card until I don't have the job letter from an employer.
It is not true, Having a job offer is not linked with this Work Permit because it is an Open Work permit based upon your provincial nomination. You can apply for Health Card Immediately but the coverage for health card comes into effect : 3 Months after your landing date into Canada. e.g You land in Canada on 1 January 2013 your Health Card coverage will start from 1 April 2013. You should have your private medical insurance for Initial 3 months. This same rule applies to you even if you land in canada on a Permanent Resident Visa.
3. Can not apply health card for my family until we don't have the PR.
This is wrong. Additional family members ( Spouse, Kids ) who are coming along with you in Canada on this FSW-Open Work Permit on Visitor Visa, Work Permit Visa , Student Visa are eligible for Health Cards , but this 3 Month wait is applicable to all of the cases.
I hope this answers your questions and helps in making your decesion regarding your move to Canada on OWP.