I wouldn’t get a pass. Reservations are mandatory on most Spanish trains and cost extra. They are also not always available online (annoying).

Check prices on renfe.com (the Spanish state railway). There are also competitors on Madrid - Alicante. October won’t be available for a long time but check January - February prices and schedules.

I’d recommend you start by having a look through this page. It’s a really useful summary of the train types in Spain and how to book. https://www.seat61.com/train-travel-in-spain.htm

You can also have a look at this map, which shows most of the major train lines https://www.seat61.com/reference/interrail-and-eurail-map.pdf

As a rough guide, you can expect Madrid to Alicante to start at about €15-20 for standard class, if you book in advance. Double that for first class.

Baggage is not really an issue unless you travel on the low cost operator Ouigo.

You can look up train schedules and prices on www.renfe.com or www.thetrainline.com . Search for a date in January or February, as @thibcabe suggests, and assume things won’t change much.

Do take into account that you have to carry your own luggage. There is no checked-in luggage on trains.