Depends on what your long term goals are. If your plan is to never have a top mount blower then the trickflow intake (street burner, not the track heat version) with a 75mm TB would provide the most power gains but it is much more costly. As far as bang per buck the intake is a pricy item for little gain on a NA mustang (~10 rwhp). With full boltons (full intake, full exhaust) you are looking at a 260 rwhp car.
With that many Boltons all at once you are most of the way to a blower kit where you would be safely in the mid 300rwhp range. If I were to do it all over again and I was looking for performance I would buy a blower kit first rather than the long bolton road.
If you still prefer to start with boltons to put on - as long as you have no long term plans to go turbo - I would focus on the exhaust and suspension side of the house for better bang for buck. Full length headers, matching mid pipe to your sound preference, and your preferred catback. Full exhaust would be about the same cost of just the intake but would easily free up 20-25rwhp.
I would skip underdrive pulleys - very little gains ..... and with the accessories spinning slower I found my power accessories were just never the same (lost some drivability ). When I swapped back to stock pulleys everything worked great again (this was all when I was NA). If you are looking for every hp possible they may work for you but if you read the forums you will see mixed reviews depending on your threshold for drivability.
