If you're going to go for it, just invite everyone you can think of. Don't worry about it being a disparate crowd. Birthday parties are often like that and people sort themselves out just fine, in my experience.
Email invites are fine for those who are online, but it's nice to put together and attach an image showing the birthday person, venue and other details, a personal message, etc. Make it visually appealing and don't forget details about eating/drinking arrangements and the RSVP. Print it out and mail it to any dinosaurs without email addresses.
Hire a pleasant, appropriately sized room for a date and time that will suit as many people as possible in your party. For food, I'd just go for a finger buffet rather than a sit-down meal, it's less formal and people can eat what they want when they want. Make sure you cater for veggies if you have any coming along.
You'll need some music - recorded is fine, but at musicians' parties I'd normally expect some sort of live stuff, even if it's just a jam session. Watch the volume, you don't want people leaving with their ears bleeding. Or maybe you do.
Encourage folks to take lots of pics (disposable cameras can help here) and get at least some of it videod by someone who can operate a camera with basic competence.
Don't lose sleep over it, just get together and have fun. We had a surprise 80th birthday party for my Dad this year, tons of folks of all ages were there and they all had a blast. It was a massive job for me to prepare for it, but that was mainly the musical aspect. The invites and basic party organisation were fairly painless.
http://www.youtube.com/watch?v=TpIhIjnv428 http://www.youtube.com/watch?v=DTwMOuZw14I http://www.youtube.com/watch?v=q1jVRv3W3mg