We've all been where you are!
Many here have given some good advice, so no need to duplicate it.
I'd add the following:
Have a couple people, friends, etc. take you out on their boat first and 'show you around'. Most people have different viewpoints and experiences that they are happy to share. They can give you some direct advice.
Then rent a boat or two for a day and try them. I did this when first boating and it helped me appreciate the size of the lake and what a smaller boat feels like on the lake.
As an example, I happened to take a friend out on the lake one day when the wind kicked up. We rounded the southern end of Rattlesnake and then turned north to return through the broads. After this trip he had a good feeling for how conditions the lake can get, and I think he changed his mind and got a bit larger boat than he was originally considering. LOL
To end the story, my boat handled the wind and waves with no problems at all, in fact it was a fun ride back! But it did give everyone a good feeling and new respect for the lake. If someone was on a pontoon at the time, everyone would have been very uncomfortable, and probably very wet!