What is the best exhaust size + brand for 1HZ in 80 seris?

I do have custom-made 3” exhaust in 80 series 1HZ + DTS turbo. In my humble opinion there is no huge difference between 2.5” and 3” (but indeed huge one over factory standard), particularly considering that you reluctant to turbo your engine. (BTW mild “turboeing” is really the best things what you can do to your 1HZ 80). Without turbo I would rather get 2.5 + extractors instead 3.0. Block who did it to me suggests running pipe between transfer case and frame (i.e. pipe not jumping over frame under driver seats like in factory installation). Best thing ever – pipe get much straighter and this contribute to better flow much more then 3 v 2.5. Place call “Professional Exhaust Centre” and located in Airport West (9338 9233) and I definitely can recommend him.

Vortex muffler with appropriately upgraded pipework or complete Taipan XP system...

Should be no probs. with the reliability of a turbo install-
if it's done properly, not a super boost (mines about 7psi), and clean oil !!

Mines an aftermarket DTS unit boosting to a max. 10lbs and its been on for 10 years and the motor currently has 395000 on the clock and uses no oil and goes great. Its a 91 model 1HZ and I am surprised you find a reliability issue. Over fueling can be just as bad for the engine so be careful winding the pump up.