I have just purchased a home with a crawl space. THe old subfloor is a type of particle board and it sags and has moisture in it from prior leaks. I am replacing any bad areas. While the whole house is currently carpeted (linoleum in Bathrooms) I am hoping to put down some laminate flooring, ceramic tile and carpet.

If I remove a section of floor down to the joists - do I need to put something between the joists and the plywood floor? Because it is a crawlspace only 12" above a dirt floor do I need special plywood. I know for the laminate flooring I will be placing an underlayment. For the tile can I just put the mortar on top of the plywood or do I need to put some underlayment there (the house is a 30 year old Manufactured home on a permanent block foundation so settling should all be done.

Any other suggestions would be helpful (I have done all of the above on concrete slab and on a house with a full basement but not on this crawlspace.)

If your crawl space is open (vented) and has a dirt floor, your best alternative would be sealing or encapsulating it. This way, you will be protecting the floor joists, and the plywood against moisture.
You will also be improving your indoor air quality and making your home up to 36% more energy efficient.

Here's two websites on dirt crawl spaces and what to do about them:

Crawl Space Guide: Create Home Energy Efficiency & a Healthy Home
CRAWLSPACES.ORG

It's probable that your sagging floor sheathing is due to soil moisture condensing on the underside of the particleboard. It sounds like moisture control in the crawl is Job One. First of all you should have at least 18" of clear space between the dirt and the underside of the floor framing so you can access this space for maintenance, repairs, etc. Then you should install a minimum 6 mil vapor retarder over the bare soil; the plastic should be lapped up the crawl walls and all laps should be taped. Oh, I forgot to mention that all debris (especially wood) should be removed from the crawl and it should be raked smooth and level. After the plastic is laid down you can throw some pea gravel on top to keep it in place. The round pea gravel won't puncture the plastic vapor retarder when you're crawling in the crawl. Then you should insulate the crawlspace walls, preferably with rigid foam insulation. It's a good idea to give your location when you post a question like this because the answers are often climate-specific. The enemy is moisture and moisture behaves differently in differing climates. Also, if you have HVAC ducts in the crawl you want to seal them with duct mastic and insulate them to prevent air losses and condensation.