I don't think is wise to just judge by looking at their portfolio through online and decide. The best is to visit them personally. Better if they have a proper organization or office. Just in case things gone wrong, you still can find the office and contact other staff. And also proper registered company or organization are protected by the law.
Also cannot judge by looking at their equipment as well. Does not mean that they use the best equipment they can produce meaningful photos.
There are such things as artistic photos and meaningful photos. Artistic photos are sometimes meaning less. They only look good as and art but there is no meaning to it. Meaningful photos are like family portraits and moments. A lot of photographers out there are too busy concentrating on artistic shots to make their portfolio look good and missed the important moments which very often are boring to others but very meaningful to you. Especially if there is only one photographer, is definitely a down side.
- Try searching for those that are working as a team. Try to avoid 1 man show. In case one of them had issues with his camera or suddenly have an emergency, like stomach ache or suddenly got sick or fainted. Sometimes it does happen.
- Like above, try to look for those licensed and registered ones. With proper organization set up. To prevent fraud.
- Get to know them personally first. Talk to them. Know their character. Know their background. Is important. You wouldn't want a stranger being so close to you for almost the whole day and have access to your house. Always preferred those who listens and agrees with your decision and ways rather than those who are so ego that they only do their own ways and achieve their own goals. Communication are very important. To be able to instruct and guide the photographer on what you really want.
- Price wise, there is really no guideline. Just need to set up a budget and choose the one that you feel more comfortable with. Don't purely judge by the portfolio. As we all know, is all filtered and massively edited.
Cheers and just my 2 cents worth............