Good for you for reaching out for advice.
Here are my opinions (but please keep in mind it's just that....)
** If you are going to have guests buying their own meals you need to make that clear on the invitation or in a phone call or email prior to the party. Likely it will cut down on the number of guests. Also, if you do go that route, make sure you call or stop by the restaurant to make sure it's OK to bring your own cake and/or ice cream. They may not allow it or may charge a cutting or serving fee to help get the cake to the table. Also make sure they can accomodate the number of guests you are expecting. They may have to staff differently to ensure quality service for such a large group.
** If you decide to go the restaurant route you may be able to work with the management staff to select a smaller menu with limited choices. You might also want to think about providing an array of appetizer choices to provide something for everyone. I understand not wanting to buy a full meal for that many people, but I feel it would be more appropriate to provide something to your guests. If you can control the cost and plan ahead, you can avoid big surprises at the end. Also remember that many restaurants automatically add an additional gratuity for large parties (typically 18-20%).
* I do not feel it is OK to ask for donations or money toward a car seat. If someone asks specifically for gift ideas you might be able to mention gift cards to a particular store. However, a car seat is a purchase your family should make. It is not a gift idea for a one-year-old's birthday.
* As far as a registry, it depends on the individual. Some people may be put off by it, while others might appreciate the suggestions. If you decide to register, don't go overboard and make sure you register for items in several different price points. Not everyone has an extra $30, $50 or $100-. Make sure there are $5 or $10- items as well. You certainly don't want someone not to help celebrate this milestone just because they couldn't afford the right gift.
* I think goody bags are a nice touch. We always give something to our guests at the kids parties. What you decide to do is up to you. Typically the gifts we give tie in with a specific theme or age. Basically it's just a little something to say thanks for coming to the event and thank you for a thoughful gift. If the people who will be attending are close to you, perhaps you could think about something a little more personal. Perhaps a small frame (or a magnet or ornament) with your son's picture. You could do bubbles or rubber duckies or a small fruit or candy bag for the kids. It's easy to get caught up in lots of dollar store trinkets, but often "less is more", especially when it comes from the heart.
* Have you thought about holding your party at a local park, playground or community center? Check with your local parks and rec department and see what's in your area. Often age-appropriate play areas are available for little or no cost. You've got an open area for guests, a safe play area for the kids, bathroom facilities and a picnic area with no cost and little effort. Just an idea.
* Remember it's also OK to invite people to join you just for cake and ice cream. Make sure you mention it specifically on the invitation so nobody shows up expecting a full meal. It is perfectly acceptable just to do dessert. Try to plan your event around meal times (don't expect people to show up at 5:00p and not feed them). Start the party in the early afternoon.
* Just a note, one mommy to another. Plan the day around your son's schedule and needs. If he has a set nap time, feeding time, etc. make sure you work that into your day. It's hard enough to entertain guests and have time to mingle. Your son will be tired and confused with all the extra excitement. Don't compound his frustration (or yours) by trying to pack too much into the day or setting your expectations too high. Just have fun and enjoy yourself.
* Make sure you send thank you notes to everyone who attends. It's just the right thing to do.
Have fun and good luck!