i just bought the reprogrammed computer at the grand prix store do i have to buy a 160 thermostat? or can i just leave like that

No you can run the stock 195 degree for the time being at least through the winter. When the weather warms up then consider going to either a 180 or 160.

U wont know wut type of mufflers to buy for my 99 pontiac gt some on a 150 budget cuz im tired of hearing my raspi sound of exhaust

What is your current exhaust setup? Are looking for a stock sound or something along the lines of Borla or Flowmaster?

It will be tough with a $150 budget.