If you plan on biking and running in your swimsuit, make sure it's a high quality suit (Speedo, TYR, Nike, etc.) that FITS correctly, so that it doesn't start to creep. If that starts to happen on the bike, you will be in major trouble with rubbing. I'm not sure how people do it anyway - I always have trouble enough with bike shorts not aggravating the crease of my legs, so the swimsuit would be a real challenge! You won't necessarily need shorts on the bike, unless you feel you want some coverage. However, if you feel that this event may be the start of a new hobby, you may want to invest in a tri suit. On a fluke, I got mine at TJMaxx for $7! If you didn't want to lay out the $$ for a new one, you could get one cheap off of eBay.
As far as the running goes, you may want to check out the Web for dealing with that Achilles injury. You'll want to find out why it was hurt, in the first place, and what you need to do (mobility exercises, massage, strength training, etc.) to keep it healthy.
Most of all - take it easy and have a ball! After it's over, you'll say 'I don't know if I'll do this again', then wait about an hour/day - you'll be hooked!
~Angie