25 degrees isnt that hot imo. so i would pack regular clothes for spring weather. 17 degrees is cold (our flat is 23 degrees right now and we are all in winter clothes). at 5 months just short sleeve vests and cotton babygros should be fine. take babygro off if warm, leave on if cold. they can be worn day/night.
i would take just hand luggage tbh. we travelled for 2 months summer 2010. with 16 month old and 2 month old. just a rucksack each and a sling for each child. (in vietnam)
it really just depends on what your happy to take. we co slept with both children whilst away, brought nappies as we went, breastfed. toys were just a small comforter and two small books for eldest and nothing for youngest. he was happy and entertained from things we found along the way. it was far easier for us as we usually only stayed 3/4 days in one area, and public transport/trains etc would have been a pain/impossible with large suitcases and prams etc
in comparison we met some friends half way and they travelled with everything except the kitchen sink. several suitcases, hand luggage, prams, beach tents, travel cots, sterilizers, playroom of toys etc. the difference being they went for 2 weeks to just one hotel. but they had to use taxis everywhere as couldnt use a train/ bus etc with prams, and had to hire help at both ends to help move it all with two small children in tow.
as your off to spain, first time with baby i would say take the suitcase, and then just small hand luggage. see how much you actually use/ what you didnt really need etc, and it will help you for future travel. its better you are happy and confident on first trip, than worried about finding something . (we are/were experienced hand luggage travellers before children also)