Hi there,
There's a previous thread entitled "How do I get from Valladolid airport to Madrid" which will answer most of your questions. Perhaps MadridMan can put a link here (or you can use the search facility) ?
I'm fairly sure you can buy a ticket for the following day - there's a counter in the bus station where you buy the tickets. The bus journey is quite pleasant as you get to see some of hills/mtns surrounding Madrid province. I've also travelled this route by train, which is a little slower as it goes via Avila. It's a very beautiful route in winter with the snow-capped mountains contrasting with the brilliant blue sky.
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An English Bookseller in Madrid