Wetsuit Question and A funny thing happened in traffic court

As I get closer to my first tri - there are still some items I need to purchase for race day. One of these items is a wetsuit.

Since I intend to also use this wetsuit in late Sept, should I consider getting a thicker wetsuit?

What exactly should I be looking for? How do I know what size I am if I order over the web?

Best brands? (Don't want to break the bank either!)

Any advice is appreciated.

P.s. - Quick story - was in traffic court the other day and it was up to the judge to impose the amount of the fine. So when I was called up to the bench, he said my plea deal (parking on sidewalk instead of going through a stop sign) was a good plea deal, but before he imposed the fine, I asked him if he could lower the amount, he asked why, I replied, "Your honor, I am training for a triathlon, and it is expensive." He laughed and started talking about his service buddy doing the IM in HI, then he lowered the fine by $50 and told me to get a real job. :-)