Will I regret wearing a swimsuit?

It all depends on what your goal for the race is.  Is your goal to finish?  Is your goal to beat a certain time? 

I would say if your goal is to finish, then a swimsuit should be doable, as long as you try it out on your bike and run BEFORE the race.

If you're trying to beat a certain time, I would go with tri-shorts.

For what it's worth, I've always used tri-shorts even on my first one.

Probably.  There is a lot of drag in a traditionally baggy swim suits.  If you have a liner, there will be some chafing issues on the run.  If you are going commando in board shorts, the bike seat may be a bit uncomfortable.

If you don't want to buy some tri shorts, I would use regular bike shorts, which will probably get a bit waterlogged and feel like running with a diaper, but with a thinner pad, should be doable.

X2... you can pick up a cheap pair for $15.
http://www.swimoutlet.com/product_p/4246.htm

My brother races in gym shorts and does sprints and olympic races. He is odd and doesn't like to conform.

I would suggest jammers though..I found some for my husband cheap at swimoutlet.com

I've done training (inc. riding) in these SwimOutlet jammer & they work fine for run & shorter (<15-20mi) bike.  Might think about ordering 1 size up though (34 waist but had to get 36 in these).