Also what varieties would you recommend?

They are worth growing for the fun! For the taste yes and no. Most people who grow giant vegetables for competitions use so many chemicals that are not food safe that you can't actually eat the plants. If you are just growing them for fun then yes you can eat them. This will help get you started https://www.worldclassgardening.com/

Alyssa Craig onion usually for the onion. For the pumpkin if you are really serious you will get a seed that has a pedigree, which means it came from a giant pumpkin in the past, and contains the genetics to get large, the best seeds will be super $$$$, but you can get good ones for $15-20 each, less for smaller pumpkins, or join a pumpkin growing club in your area and get some for free. (thats what I do, member of Wisconsin Giant Pumpkin Growers) If you just want some generic pumpkin seeds to grow a good sized one get Dill's Atlantic Giant.

Back in the early days of my attempts I grew a 275 pound pumpkin. A widow who cared for her mother said that she would take it and they would eat the whole thing. In less than 10 days she had decided it was a bad idea and scrapped it.