That's the problem. Maybe they have fields that are not available on any of the other sites. Maybe they want to run reports off the site. Let me tell you what I think their goal probably is: A few weeks before the event, they want to lock the registration, get a report of all registrants, when they get in, when they leave, if they have any dietary restrictions, who requests lodging. Then they give that to their office manager who starts contacting local hotels and caterers.
I wrote a web-based event management program a few years ago on a LAMP plus JavaScript. It's been rock-solid and has handled thousands of registrants. Having said that, here's the way to determine if their needs are truly specific (you're going to need a management module and a registrant facing module):
1) What fields do they want to display to registrants? To event planners? Problem here: clients don't know what they want till they start playing with the site.
2) What fields do they want registrants to fill in after they click on the invitation link and reach the registration page? Do registrants request lodging? Do they arrive on different days? Do they leave on different days? Do they have dietary restrictions?
3) How will registrants be allowed to edit their information until the "lock" date? Probably a combination of unique pin generated for them, plus their email address.
4) How do they want to contact the registrants and ask them to sign up? The site I created created a custom URL for each event, and they were mailed to the invitees for that event. So, you create the event, and you email all the registrants (in the BCC field) with the link.
5) You're getting PII - personally identifiable information. You need a secure server. You'll need an SSL certificate to encrypt the connection.
Fortunately, your organization has a few years of experience with this. So they know what they want to do generally. That's a very big deal - a client who actually knows what he wants.
THEN - you can check out some of the available commercial options, or see if they really need something from scratch.