Yes, any blessing by a priest is sufficient - as long as he makes the sign of the cross over it, you are all set.
But here is an English translation of an older blessing that he might use, if you ask him to (if he does not use it, but makes the sign of the cross over the medal, it is still completely, officially blessed).
It sounds like you already found this blessing, but here it is again just in case. If he does not wear the stole, that is OK. It does not invalidate the blessing. If he doesn’t have much time, he could just use the part of the blessing below that I put in bold. That’s the key part of the below blessing.
The priest who is to bless the sacred medal of the Immaculate Conception, vested in surplice and white stole, says:
P: Our help is in the name of the Lord.
All: Who made heaven and earth.
P: The Lord be with you.
All: May He also be with you.
Let us pray.
**Almighty and merciful God, who by the many appearances on earth of the Immaculate Virgin Mary were pleased to work miracles again and again for the salvation of souls; kindly pour out your blessing + on this medal, so that all who devoutly wear it and reverence it may experience the patronage of Mary Immaculate and obtain mercy from you; through Christ our Lord.
All: Amen.
**
The priest sprinkles the medal with holy water, and presents it to the person, saying:
Take this holy medal; wear it with faith, and handle it with due devotion, so that the holy and immaculate Queen of heaven may protect and defend you. And as she is ever ready to renew her wondrous acts of kindness, may she obtain for you in her mercy whatever you humbly ask of God, so that both in life and in death you may rest happily in her motherly embrace.
All: Amen.
The priest continues:
Lord, have mercy. Christ, have mercy. Lord, have mercy. Our Father (the rest inaudibly until
P: And lead us not into temptation.
All: But deliver us from evil.
P: Queen conceived without original sin.
All: Pray for us.
P: Lord, heed my prayer.
All: And let my cry be heard by you.
P: The Lord be with you.
All: May He also be with you.
Let us pray.
Lord Jesus Christ, who willed that your Mother, the blessed Virgin Mary conceived without sin, should become illustrious through countless miracles; grant that we who ever seek her patronage may finally possess everlasting joys. We ask this of you who live and reign forever and ever.
All: Amen.