Turning fantasy into reality is a huge step, both emotionally and in the planning to do your best to make sure it goes well .
I will tell you how I approached it,and maybe there will be something's for you to take from it .
We too used dating sites,indeed my wife has recently rejoined after a long absence.
Insist on talking to the guy as well,let him see you come as a team.
Reassure him you have no hidden agendas, you insist your there especially for the first time, but its strictly only your wife and him in the bedroom ( unless things evolve into a 3some as one of ours did ).
Let him know your no pushover ,tell him how much your wife means to you ,and your there to protect her at all costs, and protect her you absolutely will.
That I know can come across as heavy handed,maybe even aggressive,but I always want the guy to know how far I'd go should he not treat her with the respect she deserves.
Not to ...... the mood,I'd also talk to him about how much I want him to enjoy fucking my wife,and how when the time comes, I'd always tell him she's no longer my wife ,she belongs to him to fuck as he wants , that may sound a bit much, but I know my wife likes the idea of being somewhat dominated in the bedroom.
My wife has been with 6 men so far, all but one happened in our home.
I found the biggest concern the men had was the awkwardness of me being so close ,so getting them to trust you will not disturb them is a big thing ,they always ended up worrying more about me than I did them, but that was intentional.
I found convincing them it's what you want, even letting them know they are doing you a favour by fucking your wife senseless always seemed to turn them in even more, I would play on knowing most men have this desire to take another man's wife off him and give her a good seeing to ,especially the desire to get their seed inside another man's wife .
We almost stalkers lol,spending some time getting to know the guy,tracking him on Facebook etc,asking him questions about his past etc.
We tried to get to build a picture of his past, especially his sexual history , and if it turned out that he was likely ...... with different woman on a regular basis, we'd not proceed.
The more you plan beforehand ,the more open you are with the other guy,the more you explain the rules and openly discuss what each of you want from it,the better the meeting goes.
I left it in no doubt that it's a win win win situation, happy that my wife wants him to fuck her, happy that he wants to fuck my wife,but putting him at ease, maybe turning him on more by letting him know how much I want him to fuck my wife , that always seem to put everyone at ease.