Originally Posted by
tafi .
Whether you like it or not, you will need to be able to turn the big gears to keep up (let alone compete) in racing. You will know your tri courses better than I do, but the vast majority of tri bike courses I have seen have either been pan flat or gently rolling. So there is no reason at all why a standard 53/39 crank and 11-23 cassette shouldn't suffice for this. Since there is no need to sprint and probably no long downhills, you may be able to get away with a 12 tooth small sprocket and therefore use a 12-25.
The whole point of the standard crank is that you will have access to the right combinations of gears on varying terrain at
racing speed. If a course happens to be a bit hillier then you will also be able to use a 12-27 or 12-28 cassette with the normal derailleur.
The compact crank is not designed for racing, but for recreational riding as a replacement for the older triple crank. Recreational riders aren't as likely to use the same big gears required in racing and they also aren't needing to get over long climbs at 20-25km/h, so they can save the pain by chosing a generally lower gear set (smaller chainrings). The only time a compact has been usefull in racing is on particularly hilly courses (I mean big f*ck-off climbs) where climbing speeds are low enough to make it useful.
In my view, if your aim is competition, it is better to choose a racing crank and swap out the cassette to suit the course, than to hamstring yourself with a compact crank.
I'm personally no great shakes as a racer but all of my racing and training over the last 10 years has been conducted using a 53/39 (or 53/42 if fit) crank and an 11-23 cassette. This comes down to the simple reason that, if I can't push these gears over the climbs, then there is really no point in me turning up at the start line at all.
Cue flaming
.....
As for choice of parts, I would proably go for new stuff. You are new to building up bikes so it is best to minimise the chances of hidden "surprises". Unles you're certain of what you're doing, buying used can end up costing you a lot more in the long run.