You need to mention the location of your construction project to get answer of your question at sr. i & ii above.
As regards to your question at sr. iii above:-
Ambulance at site:
You need to ensure Ambulance van at site according to Section 227 of Central Rules 1998 as applicable to you. This section says that you can make arrangement with nearby hospital for providing ambulance van for transportation of serious cases of accident and sickness to hospital promptly. There is no criteria of number of labours employed by you for ensuring Ambulance van.
Appointment of doctor:
You need to ensure an occupational health centre and a construction medial officer at the occupational health centre possessing qualification as laid down in Schedule XI, if your work is involving hazardous process as specified in Schedule IX. There is no criteria of number of labours employed by you in this case also. Please refer the rule 225 under the Central Rules 1998 relating to this aspect.
Health check up:
You need to ensure medical examination of your labours periodically if the work involving of such labours of risk or hazards or the labour is engaged as an operator on crane, winch or on other lifting appliance, transport equipment or vehicle. This medical examination is to be done in accordance with Schedule VII and conducted by medical officer or at hospital as approved by Central Government. This medical examination shall be arrange before employing labours on above said work and after illness / injury to them affecting their fitness and thereafter, once in every 2 years up to age of 40 and once in a year after 40. Please refer Schedule VII.
Similar provision you will find in States Rules also.