JT. Tell your daughter your concerns & why. But let her know you recognize she is an adult & can make her own decisions.
Tell her you'd like to see a recent photo of this guy before he arrives & that you'd like to meet him. That you only want to see her happy. Its about all you can do. Unless you get the sense she might be looking for you to tell her she's being a bit crazy? Is that your sense?
I think its pretty normal for any father to want to break the legs of anyone who might hurt his daughter. Mine used to threaten to drown my potential BFs in our pool by asking how long they could hold their breath. You know, joking with them but... not really. LOL.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh