This is hard and only a choice you can make.
However, its going to be really hard to find that perfectly trained horse. There are a lot of horses out there in various price ranges, but very few are truly trained and if they are - I find their price reflects it. Now occasionally you can get extremely lucky and find a really well trained horse on the cheap, but then again these are usually oldies (but still a great option for a green rider!) or maybe having soundness issues.
Personally if I had a horse that was already a great trail horse - I would hire a trainer to take care of those other issues. As I see it - you can hire a trainer to fix the problems on a horse you are familiar with and know, or you can go out and spend the same extra money on a better trained horse and still find you have issues you weren't expecting.