The forward seat post effectively increases the seat tube angle (78 degs is the angle favoured by triathletes) so the whole body 'pivots' round the bottom bracket.
This is also supposed to make running off the bike easier as the leg muscles are used in a slightly different way.
I would suggest going with the frame from your dad, as a) it's always fun to build up a bike, b) it's sitting there not doing much anyway so it will be a cheap buil, and c) it might well work! Plus even if it isn't perfect you will most likely be able to sell it on to a tri newbie as there are so mnay people coming into the sport.
Best of luck - just my $0.02
