If your floor boards are really solid/stable, fix a 6mm solid cement board (Hardibacker), or if you want to save money longterm, use a 10mm insulation tile backer board (Jako). The insulation board will save you money as the heat from the UFH will go up, not down.
As above, I would use a 5mm screed of levelling compound over the top of any boarding once the UFH has been laid, as it encapsulates the UFH and allows for easier and a better tile bond with reduced chance of damage and good heat transmission. Screeding can be difficult for an amateur, so I would advise asking a tiler who has done this before.
Adhesive is fine. Any S1 adhesive will do the job, but mix it properly and use
standard set if you are new to the job. Don't forget to prime floor substrates if you are using levelling compound as it can dry and pin-hole bubble/crack.