better wood for a plinth

Maple

Jatoba (brazilian cherry) was not on his list, but would do nicely as would Santos Mahogany- both are very hard and dense.

I am not familiar with working with Eucalyptus, but shy away from thick oak boards. Laminate if you need to use oak as it often has internal cracks and/or voids from the drying process that you may never see. It is a common problem with 12/4 or 16/4 oak.

I have found a table of oak of 5cm of thickness (2 inches), is good?

I built a tiger maple plinth for my table. I used Herbies washer between the plinth and the turntable chassis. The washers are in each corner. This combination works very well. I would recommend the washers, regardless of which type of wood you decide to use.