If you are planning on reinstalling the timber as a floor, then leave the varnish on, reinstall the floor (maybe a rebating plane – hand or power can remove the varnish from the tongues and grooves) then have it sanded and polished after it is installed.
If you machine it now then install it, you are still going to need to sand it to take out any proud edges and you don’t want to take off too much timber.
If you just want to have some nice timber to use ’some day’ then maybe a coarse grit belt sander across the grain then a fine one along it will work (you also need time with this option), although this could cause rounding on the edges…
Or maybe bite the bullet and buy a thicknesser/bench planer… or look into hiring one? a thicknesser would be a better option if you are going to reuse it as a floor, this will give you a precise thickness across your pack of timber, a power planer may leave little grooves (as a result of only having 2 blades) and will need to be sanded after.