I have had four Spanish au pairs, female, male, female and (currently) male (hereafter referred to as f1, m1, f2 and m2). All have been very respectful, not intrusive and only f2 was hard work to get along with (she meant well but I think she was bit homesick and couldn't settle, although she said not when I asked gently). It's not as weird as I thought it would be to have a young man around, and the DC have both preferred the two male au pairs.
As a wild generalisation the men have been slightly less good at housework, they will do all the tasks that I tell them about (empty dishwasher, wash DC's clothes, lay and clear table etc) but are less proactive about seeing random things and doing them. However I don't mind it as f2 was too far the other direction and it drove me a little insane, for instance I would put my hand out for my last mouthful of coffee and the mug would already be in the dishwasher.
All of my au pairs have been mid to late 20s, partly for car insurance cost reasons and partly because I think they are much more independent and don't need looking after much at that age. They all seem very sociable and would tend to go out with their friends in the evenings and at weekends (f2 was a bit shyer but still had friends and went out).
I looked for people who had volunteered or worked with children, for instance as a summer camp leader, coaching football or something where they had to wrangle unruly children and get them to cooperate happily. Having minded their nieces or nephews isn't enough IMO.
I also looked for people who like to travel and had either travelled extensively to different countries or had lived/studied away from home. I felt they were less likely to get homesick and more likely to settle into a new, strange routine with ease.
This is personal to my family, but I need someone who is very fit and loves sports and activities because my DC are lively. You might prefer interests that coincide with your DC's interests, eg arty/crafty.
When you have a short list of likely sounding people (at least 3 or 4), do Skype interviews. I have never ended up employing the one who sounded best on paper, they just didn't seem the right fit when I spoke to them. It's worth letting the DC chat to them for a while as well, if they can engage the interest of the DC over the internet then it's more likely they will get on well in person.
Where are you based? I had two excellent short-listers who were looking to come to Ireland, but the timing didn't suit very well.