buy the Keukenhof combo ticket which includes express bus transport and admission ticket. If you are near Amsterdam Central station, take the train to the airport. At the airport, exit the plaza and go outside, turn right and walk 300+ feet to where the Express bus leaves. There will be Keukenhof/bus personnel there to check your ticket and put you on the bus. Since you have to decide on a bus time, the buses are not overcrowded as they limit the number of bus tickets per bus. And the buses are frequent. You have a half hour leeway (before or after you booked time) on the bus time. So, for example, we had 10:00 am bus tickets, but got there at 9:30 am and they put us on the bus.
The combo ticket is round trip, so when you are finished at Keukenhof, you take the bus back to the airport, and then the train back to Amsterdam Central station. Couldn't be easier.
The time reservation is only for the bus. Your admission tickets are good for the day anytime.
A small group tour is not better and I wouldn't pay extra for it. And a tour isn't needed. You enter the park and you walk around amazed at the quantity, the arrangement and color of the tulips. You really don't need a guided tour to experience Keukenhof. Frankly, what's the tour guide going to tell you? ... This a blue tulip... there is a yellow tulip, oh, a red tulip....... . If you really need to know the type of tulip, read the signs telling you the name of the tulip.
But if you feel more comfortable having a private company drive you to Keukenhof, then by all means go ahead and spend the extra money. It's your trip. Enjoy Keukenhof!
PS. If you need more, here's a YouTube Video recording how to do everything I just wrote above, if that makes your journey easier and better.
https://www.youtube.com/watch?v=YUNlKFApvJY