Best wetsuit for beginners?

If you're going to race those distance, I wouldn't say that you're a beginner.

My advice is to try lots of them on. Some fit differently than others, some are 2 pieces, some are sleeveless, and it's more a question of comfort than anything else. And the sizes are just difficult to work out from a catalog. If there isn't a store near you, then I'd speak to a customer service rep and get a few sent out to you, returning the ones you don't like. It would be worth the return shipping cost. Believe me, chaffing in salt water is not fun.

Or, if you have some open water races coming up this summer, see if you can rent a few different suits for a test drive. Some races have links on their sites for rentals.

As for me -- I have an Ironman Stealth, which I bought at the end of a season, so it was about 1/2 price. That's the other suggestion I have for you. Just like with cars (at least in the US), wetsuits are cheaper in September than they are in March. So if you don't absolutely positively need one this summer, you might save some serious $$$ if you wait until fall to buy it.

Good luck!

All things considered, if the thickness of the wetsuit is the same across models, there's probably no noticeable difference in bouyancy.  Just get something that feels comfortable on you in the water.

Buoyancy is 100% about the mass of the neoprene vs the mass of the water is displaces. There's very little difference in the density of the various neoprenes out there, so for a given thickness, you're going to see essentially the same amount of "floatiness". The main differences are going to come in how the suit fits. And that is the most important part of buying a wetsuit -- how it fits you. Most companies will have suits at different price-points. Find the one that fits your budget and body.