I was one of the organisers of our ten year school reunion last summer.
We kept it very relaxed and low cost (most of our schoolmates now at stage of getting married/ having babies so low cost was key to getting people to attend).
Sent out invitations around three months in advance, we asked people to RSVP by text or email and told everyone it would cost €10 each, but didn't need any money in advance.
We also asked people to email old funny pics and stories. On the night, we had a big presentation of old photos, concert tickets, timetables, etc going on a loop on a projector all night. We also had a big banner and lots of pictures posted up around the room. We just had finger food and piped music from the pub upstairs. (We rented the function room downstairs.)
There was a bit of an issue as to whether to invite partners or not, so we didn't specify either way on the invitations, but only 3-4 people brought partners...more brought them along at the end of the night.
Was a really great night, everyone seemed to really enjoy it, and the relaxed, no fuss atmosphere added to it. I think had we to make it a formal meal in a hotel with advance booking and all that, it would have turned people off or made it less fun.
Money was an issue, 2 of us did all the spending, and with charging everyone €10 on the way, expected to make a profit at the end of the night, but we actually ended up making a loss..was only about €30 loss so not a big deal, but still a good idea to keep an eye on costs. Printing, posting, envelopes, decorations, etc all add up!
Good luck with it!! I'm sure it'll be a great success!