Many properties from the beginning of the 20th century do not comply with current legal rules. For example, some of them do not even comply with the urban development plan and once you purchase the property you could receive claims from the local authority to make you demolish part of the building to adjust to that plan.
Other properties haven’t passed the technical building inspection (ITE) and there could be fines for that. Be aware that as well as looking for an architect to solve this problem, (which will most likely be the highest expense to get all the necessary work done) you could face any fines that were not paid by the vendors.
It may also be the case that the building has not been divided into any separate plots for the purposes of the horizontal property laws – or often it has only been done half way through, such that there´s been a division at the Land Registry but not in the Catastro (another public entity involved). So this may be a further process that may need to be carried-out after the purchase of a building or you will not be able to sell any individual apartments independently. Most of them don’t have a community of owners constituted, which you will need in the future, especially if you are not living in Spain.