Decathlon has this for about 300 quid - gives you an idea of what you can get for your money
if you buy a new bike.http://www.decathlon.co.uk/triban-3-red ... er_opinion
Otherwise, the second hand market is worth a look, although triathlon specific bikes won't be cheap.
Have a look at the for sale section here, or ebay, gumtree etc.
You'll probably want something with STIs/Ergos (gear shifters integrated with brake levers).
You can get triathlon type bars that clamp on to the normal handelbars for a bit more aerodynamic positionining.
If you're 6 foot, a frame somewhere between 56-58 cm (length of seat tube) should be right for you.