General questions and discussions.

You always pay for modifying a spell. If you don't lock it in immediately, you can lock it in at any time later for free. Modifying it, however, always costs. In terms of cost, researching a powerful spell or a town portal spell is no different from rebuilding (a town portal spell is even cheaper, as you don't need to build a Guild Hall 5, plus you can research it in parallel with construction, as Lizard mentioned, on the same day as building the Guild Hall). But when researching, you won't get the intermediate spells into your spellbook; you'll only get the final powerful spell or town portal spell (and whatever was there at the beginning). In the case of a town portal spell, however, you can lock in a Berserk spell and continue researching in the second slot. The same applies to the library in the Tower. But you won't be able to learn all the spells in the game from a single Guild Hall.