You normally use wood primer on new/unpainted wood, then undercoat, then gloss/eggshell etc.

You do get primer undercoat paints, 2 in 1, quick drying.

I would paint the wood with a water based undercoat (or the primer undercoat mentioned above) then paint the emulsion onto that.

There are varnishes that will do the job.

You could paint the wood with satin or eggshell (as opposed to gloss), in the colour you want and not bother with the emulsion and varnish option.

Hi Josie.

Don't know what sort of distressed finish you are going for but as it's tongue and groove I would imagine it would be a 'dragged' finish.
Forget water based paints, ie, emulsion or acrylic, as the paint you use for distressing will soak in and start drying before you have had a chance to play about. Use an oil based eggshell as a base coat, this will ensure the distressing will stay workable for longer, any mistakes you then make can be wiped off and you can have another go.

The other colour you will be using (if any) should be eggshell as well.

Eggshell is self undercoating so you can apply two coats on top of the primer, rubbing down between coats.  Eggshell dries with a sheen so if you really want a matt finish you can buy a water based matt varnish to finish with. Being water based it will not discolour. When you put it on it will go all 'milky' but will dry clear.

In that case Josie, use a flat oil paint and then there will be no need to varnish.

Let's get this straight Josie, you sanded down the old painted T & G, primed it, probably going to give it two coats of paint and then sand it to make it look like... old painted T&G.  

Just kidding