Granite tiles of different thicknesses

[fredmila]

Can people recommend what are the most economical materials -that will do the job- and method to lay granite tiles on a house ground floor (dining, kitchen, etc)?
The substrate is a sand/cement screed with underfloor heating water pipes in it. It is dusty but level. The screed was laid a few months ago so it has had plenty of time to dry (it has gone through a summer).
The tiles vary from 18 to 22 mm thick, size 30 x 60 cm. They need sealing because they are quite absorbent. They are polished and are light in colour.