You've not provided much info. Will you be trailering your boat or do you have a nearby marina with docking facilities? I assume you or your neighbors don't have private docks on the lake. How do you plan to use a boat? Fishing? Pleasure cruises? Towing water toys?
With the advent of tri-toon boats, more and more people are using pontoon boats on the Great Lakes, but honestly, they are more of an inland lake or river boat. Because of the wind and waves, there are fewer decent boating days than on inland lakes, too.
Boating on such a lake requires adequate boat length, motor horsepower and a well-trained, experienced operator. So without much to go on, I'd say a minimum would be a 24' tri-toon with at least a 200 hp outboard.
If I was in your shoes, I'd start poking around the neighborhood and local marinas and start asking lots of questions. Find a pontoon owner, buy 'em a beer and ask specific questions about their boating experiences. Once you know what's actually possible, from a fellow boater, start asking about nearby dealers. I think most would agree that having a local, honest and seasoned dealer/marina may be even more important that the brand of boat you choose. Of course, if you want the best, choose Bennington.
PS: I'm not talking out my stern here. I've been boating on Lake Michigan for 30 years with a 24' stern-drive cuddy-cabin boat. I've owned Benningtons for 10 years. During that 10 year period, I took the pontoon boat on on the Big Lake exactly 1 time. It looked smooth, but once I got on the lake, it was lumpier than it looked. After about 15 minutes, I turned around and headed back. I usually boat with a crowd, but was alone that day. I know without a doubt that had I had passengers aboard, they would NOT have enjoyed themselves.
You'll hear different opinions, I'm sure, but this is my informed opinion. Good luck.
