If a wood porch - be sure not to put it so close to the ground that it becomes an animal/insect haven, or can't be cleaned out under - I recommend minimum 12 inch clearance under the joists for those reasons and to allow for inspection and repair, as well as cleaning out weeds and leaves.
Treated wood almost certainly cheaper than composite - nominal 2x6 for instance runs typically just over $1/LF - $2-3/LF for composites and Trex is typical range. Plus the supporting timbers shjould still be wood, not composite - and treated lasts a LOT longer than untreated, especially if the top edge where the decking contact it is protected - I use a strip about 1" wider than the joist of ice and water shield to act as a water and debris barrier to protect the joist from debris buildup and water (droops over edges to act at dripedge).
Cost depends on your particular situation of course, but ballpark - $10-40/SF for normal deck with very simple code-compliant railings and one set stairs - typically about $20/SF for low treated wood deck, rarely as cheap as $10/SF except on DIY jobs or decks just built on grade on concrete pier blocks, with no structural connection to the house, which is what I would recommend for your case to minimize issues of rot where the deck would otherwise connect to the house.
Generally, if the grade is pretty flat in the area you want to build it, a concrete slab or concrete pavers will be cheaper than a wood deck - and certainly in the long run due to deck retreatment/staining costs. Concrete slab generally, assuming little subgrade prep required, about $5/SF plus or minus a buck or so.
