The biggest thing is to always rinse the underneath of the car with fresh water after every single time you go on the beach. Rinse through any underbody trays, and into any holes in the chassis where sand can get into.
As far as tyres go, standard road tyres are fine in the sand, in fact they some times work better than A/T's and M/T's. The biggest thing is having a decent tyre that you can let the pressure down if you need to. Perhaps get some smaller wheels and then tyres with a bigger sidewall. Letting the tyres down in pressure makes a world of difference and reduces the load on the transmission. Always pays to carry a mobile compressor on board so you can inflate them again as soon as you get back onto the road.
You will be surprised how well it will go on the sand. The Subaru AWD system is very good!!