i have a older g-shock that has a count down timer on it and it beeps when it gets down to zero (i think it even beeps at 20, 15, 10 & 5 sec. can't remember for sure). gshocks can get a little pricey, but i'm sure someone makes a watch like what you're looking for under 50bucks.
shoot, if i can find it i'll give it to you. proly just needs a battery.
For my triathlons I wear a Timex Ironman watch with a hr monitor for the whole race. I like to have the hr monitor on during the races. I am sure it has a countdown timer on it and I need to take a look at that for nutrition purposes before my Timberman HIM in 3 weeks.
If you are used to looking at your hr while using your Garmin you may want to look at that. I think mine was around $80.
Another vote for Timex Ironman. It's my everyday/ training/racing.watch. It's my.alarm clock.too. Very reliable and almost indestructible. I have joked it will continue to emit it's shrill, piercing beep from beyond the grave as I will likely be buried in ir.