Hi Johnny, welcome to the One Ring - oh, and this is the right spot for your question
Sand is a good base for a gaming table and if you have it available in large amounts then that's what I'd recommend you use. Using anything else can get expensive if you've a large area to cover.
PVA (Elmers / White Glue / Wood Glue - it's all the same) is a great way to stick sand to the table surface (and your fingers). But use a kitchen sieve to spread the sand out over the glue - you'll get a lot more even coverage that way.
Painting that much area will take a lot of paint so it's best to use cheap acrylic paints from your local craft shop instead of the hobby paints (like GW or Vallejo). I usually go for Burnt Umber as the base colour, which is a good match for GW's Graveyard Earth, and then use a yellow ochre to drybrush the raised surfaces to give it a bit of life.
I'd probably cover the sand with some patches of flock and static grass to make it more attractive and a better match for your models' bases, but you can leave taht step until you've got a bit more cash - can be a bit expensive. (In any event, you can get much better value for money using flock and static grass from a model railroad store instead of GW's stuff.)
As for elven buildings - that's a big question hidden in few words. Elven architecture is probably the most complex of all building styles in Middle-earth and making them look "right" is a bit tricky for someone starting out in the hobby.
I'd recommend you start with something small first, maybe a 'flet' (a hidden platform in a tree) - the extended edition of the Fellowship of the Ring has one in the Lothlorien scene.
After that you could do some statues or ruined arches covered in moss and ivy, and work your way up to doing full buildings.