Buying sifrei kodesh (holy Jewish books) should take priority over the mitzvah of writing a Sefer Torah. Only after a person has purchased all of the sefarim they and their family need in order to learn Torah properly, should the mitzvah of writing a Sefer Torah be undertaken. In case a person had one Sefer Torah written and wishes to commission a second one, the halachah is that they should spend the money on hachzakat lomdei Torah (supporting people who learn Torah) instead.
In fact, although it is written that the culmination of writing a Sefer Torah should be celebrated with a seudat mitzvah (celebratory meal) and with a lot of fanfare, it should not be overdone. Since in some ways the mitzvah of supporting aniyim (poor people) who sit and learn Torah is greater than the mitzvah of ketivat Sefer Torah, it is certainly preferable to make a modest, proper celebration instead of a lavish one, and donate the difference to support indigent talmidei chachamim (Torah scholars).