:welcome:
As you are fitting a new kitchen take into account the extra height that you will need for the ply and the tiles! kitchen units are normally installed at 870mm from floor or 910mm including worktop if using a 40mm work top!
I would add on 12mm for ply and 10mm for
tile if its this thick? and then say 5mm for adhesive and this will give you enough clearence for your appliances to slide back under the counter tops.
As long as the wooden floor you have got is solid (no bounce/deflection etc) then 12mm ply will be fine!
(if its a wooden floating floor then this is totally different!!!!)
Firstly make sure the existing wooden floor is securely fixed, then i would use WBP ply or similiar marine grade say, i would seal the back and edges and then screw it down to the floor at a minimun of 300mm centres 200mm centres for extra piece of mind!
I would use a single part flexible adhesive to lay the tiles with! Look on the bag to see if the face of the ply needs sealing first! Some adhesive manufacturers recommend it and some dont! If it needs sealing use an acrylic primer! NOT PVA!!! I would then grout it with a flexible grout as well!!
I would
tile the floor upto the kitchen unit feet and then fitt the kick boards/plynths afterwards this will give you a lot better finish and if you have to remove the plynths at a later stage it will be easier to do so!
Hope this helps
p.s mapei,bal,are the better adhesives depends what the
tile shop has in stock.