+1 on figuring out fit first, so that you can get a bike that not only fits initially but has plenty of room for adjustment as you get used to TTing and your position evolves. Assuming there will be a triathlon club near you that might be a good place to start in terms of finding a recommendation for a good local fitter.
In terms of bikes, there are some very good older frames out there - something like a Cervelo P3 for example can be built out so that it gives very little away to a modern superbike (assuming you fit well on the P3). The trend over the last few years has been ever-increasing integration, particularly around brakes, stem and head tube and bars. Problem is this level of integration has brought aero benefits but in some cases at the cost of adjustability and ease of maintenance, particularly on some of the earlier superbikes (new ones are better but expensive). If you go secondhand make sure you get something that has plenty of adjustability and isn’t a compete PITA to work on.