Which one ?
A second hand one would be my suggestion. That's how I started. Like you I did a Try a tri on a mountain bike and loved it,so bought a second hand Specialized Dolce to try road cycling. I now have an Axial Cube WLS after a bike fit
I've heard a lot of people compliment the decathlon bikes for people new to road riding. Don't be afraid of cleats - make sure you have them set at their most loose and practice clipping in and out with both feet (separately) before you get on your bike - then to only thing you need to remember is to repeat "cleats" to yourself everytime you slow down. Be prepared to fall over at least once, just try to dow it to the kerb side.