there is no point deregistering ur car for a yellow sticker. goin to cause you more hassles later. you still have to go through the pits anyways.
yes it costs $80 something to take your car through and they check everything from ur parklights and license-plate lights operating properly, to rust, to steering boot cracks, ur seat belts, ur heater system, ur suspension (how stiff they are), ur tyre wear, and oil leaks.
its pretty much luck of the draw.
consensus is not to go welshpool but try midland or oconnor. dont bother going on a friday cuz every backyard mechanic trys to pass his car that day. rather pick middle of the week like tues or wed.
i'd run through the car, check for leaks, cracks and stuff, and just try ur luck. if u fail u fail. but atleast u know exactly what needs to be fixed.
goodluck.