Exhaust.

Tiapan or Beaudesert both seem to be good products. I have 3" Beaudesert and very happy so far.

Have a good look at the standard exhaust. It is a very free flowing and maybe it is not worth spending the extra $1000 + on a new one.

I believe Beaudesert exhausts found 2 3/4" was the best size they could find for power gains.

Have a look here, this should give you plenty to read about regarding bigger exhaust on the BT50/ Ranger. Their is 6 pages on the King Brown exhaust, and i am sure if you put the other exhausts in the search function you will get what you are after.

King brown exhaust link

Exforce 3" stainless.

The 3.2L 5 cylinder engine pulls well and the vehicle drives well.
Isn't the power, which is more than the competition anyway, enough, or do you want more and more for a reason?

If you're not worried about engine braking for for hilly areas depending on where you live and you're more interested in getting as much power as you can from your exhaust go the 3 inch but you'll be on the brakes a lot more. Other the that I believe a 2 1/4 or 2 1/2 would be adequate and would still make a big difference over the standard system.