I've been looking for one that my 19month old will be able to play in safely in the back garden, preferably with a lid, sunshade and a bottom to it (many are advertised as having plastic sheeting only).
got ds this at age 1.5. It's cheap, can be moved in winter to shed, takes 3 bags x10kg sand and he never spends more than 15 min in it as he is carting the sand around the garden with his wheelbarrow and spades etc. anyway.
I just felt to give up lawn space is no option so it sits just outside our patio door. Instead of using one half for water we use it to cover in rain etc.
Another reason we didn't go for the plastic underlay was that if it gets wet, it never really dries out, we drilled 2 tiny holes in the plastic one and the water drains out (which means ds can play with a watering can inside it).
Wooden bottoms tend to rot over time so it is best to go with a membrane however if you really want a floor have you seen the picnic table versions?? I got mine for my little girl - she loves it as the lid make the table into a proper picnic table and its nice to be able to cover the sand and put it away I got mine http://www.sandpitshop.co.uk/acatalog/Garden_Games_Picnic_Table_Sandpit.html - I think they also do a more traditional sandpit with a sun shade that you might like but I think that has the underlay so you might not like it????