I bought an Avant Kona second hand and I'm curious as to whether I can put drop handlebars on instead of the aerobars it currently has. I like the frame so don't want to change that but just wanting to know if it would be possible to them and if so what would I need to do so. Thank you
Yes you can install drop bars on any bike.
As to what else you need to change; you have to provide us with a picture of your bike and its specs.
Google turned up nothing on a "Avant Kona".
1. keep the bar-con shifters and put at the end of the drop bars; need bars, brake levers (and I would get cross-levers in addition to the regular Aero levers) - in the USA could do it for $100- with all parts from Amazon, including new cables.
2. change to brifters ($$$); I would still add cross levers (my 310 has brifters with cross levers).
I dislike cross levers, they make no sense to me - but then I do not ride on the flat portion of the drop bar (only when climbing, and then only part of the time). For me they only made the brake action a bit mushy (but that was on a bike with difficult brakes anyway...)
The issue you might run into is that a tri frame has a different geometry than a road bike. You cannot turn either into the other. The problem is that they are designed to be ridden in very different ways and when you change your position from one to the other (aero to road or vice versa) you change the weight distribution which changes the handling.
Edit: Nigels comments are of course very valid (apart from the cross levers in my opinion)