The hall and bouncy castle are your biggest expense, and they'll be the same regardless of how many you invite. I'd invite everyone, you always get people who cancel on the day with sickness, plus those who can't make it to begin with.
Choose a time either in the morning or later afternoon where lunch won't be expected. Cuts down on food cost and waste. Just get crisps, biscuits, sweets and loads of drinks. You can get a big tray of water from Costco for less than £5.
Parties are about treat food, so don't worry about fruit, etc.
Don't bother with the book. A lot of thought for you and tbh most of the books we've been given at parties we already have. The party favours my kids like the best are sweet cones or a bag of haribo and a piece of birthday cake.
My eldest is in year 6 and has already had a couple of class parties this year! They seem to have made a comeback for the class as a 'last party' thing which is nice.
Keep the party to 90 minutes and don't stress the kids will run wild, eat treats, drink and have a great time.