Will this wetsuit work for a newbie?

I am training for my first organized open water swim - the Oxford Belview Sharkfest in Maryland this May. Water temperatures are predicted to be high sixties to low seventies. I have a lifetime of experience with recreational swimming in muddy lakes and rivers as well as scuba diving, so the open water part doesn't creep me out. My masters coach feels I will be ready conditioning wise based on pool workouts. My goal is to do the 1500 m in under the 40 minutes allowed and to place better than last place in my 50-55 age group. My question is whether I can get by with a SCUBA wetsuit I already own. I'd rather not spend the money on a triathlon suit until I know that this sport is for me. I presently have 0.5 mm full length suit that I often combine with a 2 mm vest, and they have served me well for SCUBA diving. I've been considering getting lavacore shirt to go under or upgrading to a 3/2 SCUBA suit (which I know will get much use). Would any of these work for a first open water swim for a person whose goal is just to finish without embarassing myself? Any advice would be appreciated