In making the invitations I just used regular cardstock and punched circles out (they were less than 1 inch) and attached them to the front with foam tape to make them stand up a little. I printed the inside on white using red, yellow, green and blue writing. I actually had a hard time with the blue and green cardstock. Believe it or not I did not have these colors in my stash. I did end up finding some textured cardstock to use.
Decorations were easy because I just used cardstock pieces with circles and foam tape to represent Lego blocks. I bought a circle punch for this-it was a one inch size. I also bought some blue and green cardstock so I had primary colors. I set up a banner on our front closet, lego pieces on the stairs and pieces hanging from our light fixture in the dining room.
I just used solid colored plates and napkins in the same colors as the Legos-blue, red, yellow and green. I was going to wrap juice boxes with cardstock and put circles on them to make them look like a Lego brick with 2 studs but Brian bought Capri Sun instead of juice boxes. One less thing for me to do.
I bought a small gift bag set at Michaels and used the circle punch to add studs to the bags. Inside the goody bag I made lego shaped crayons using a mold that my sister-in-law Lisa had. I also used candy melts and another mold that Lisa had to make Lego candy. I made Lego heart necklaces for the girls (Natalie LOVES wearing hers) and Lego magnets for the boys. We also had N2 design cars with various Lego pieces and then we purchased the pieces from Bricklink and made up Lego kits for all the kids. We used those Lego pieces during the party. I downloaded the Lego font and printed up tags for the candy, crayons and cars on labels and stuck them to bags. I also printed up directions for making each of N2's cars and included those in the goody bags.
What I would suggest is go to a Lego store, if you have one by you, take a look at what they have in terms of pieces. Buy however many (I made 14 sets) of each piece that you think is interesting. You have to buy a cup of Legos-you can't just buy individual pieces but I am going to assume that you will have plenty of Legos to fill up a cup. You may need more than one cup.
If you don't have a Lego store near you or you can't find wheels or other parts you want at the Lego store, go to Bricklink and search for a piece or two that you are interested in having in your kit. Make sure that ONE seller has all of that one piece that you need (so if you need 14 sets, make sure that one seller has 14 of that one item). Then search that seller's store for other pieces that you can use for the car kit and that they have the right number of in stock. You may not get them the cheapest but it will be the easiest! I would probably start with the wheels and hubs. Not all sellers have wheels and hubs but most sellers have bricks and plates. You may need to order from more than one seller. But just buy what they have in stock. If they don't have the right number of pieces, then find a different piece to use. Then have your child create the car. I think that way would be MUCH easier than the way I did it!
For N2's family party we didn't do any food that related to Legos. We just served Mexican food-quesadillas, tacos, beans, corn, fresh veggies and fresh fruit because that is what N2 love the most. For their friends' party we had pizza from Costco. I put on pepperoni and had Brian slice the pizza to look like a Lego brick with 2 studs. Most kids picked it off but whatever it was fun! Since I didn't get to do my juice boxes to look like Legos, I wanted the pizza to look like Legos!