I'll leave the varnish experts to make recommendations on brand, but what I
will add is this:

If you have sanded the floor, then you have sanded off the very compacted,
hard top surface. The wood underneath will be quite soft, and will dent
easily as you move furniture around. It may also crack any varnish coating
you use.

So, whatever goes on first, has to be very thin and be able to penetrate the
wood to make it really tough. I've made the mistake of not watering down
(with whatever solvent) the first few coats sufficiently, and the floor
looks terrible quite quickly as the soft wood gives way to constant use.

-- JJ