How much HP gain?

By doing the PI cams, intake and heads first, you would realize more gains from the intake and exhaust bolts ons you have listed. I would do the long tubes at that time also just for simplicity sake.
I have a 2002 auto and started at 208 rwhp. I already have PI cams/heads and most of the bolt ons you mentioned. I've inquired about the TB, plenum, intake and LT's and was told that would put me at maybe 260 rwhp. So that's pretty much where I think you'd end up. Without the PI, your bolt ons would yield minimal gains.

Understand, This is based on my own experience. I'm no mechanic. Lots of inquiries to people that know. So I hope it helps.

PI's first. IMHO. With just those, you're going to notice a big difference. Then the additional bolt ons will be more noticeable also.

Some of those items come off when you do the PI headswap and some are easier to install when it's all apart or you would be reinstalling when you do the headswap i.e. LTs and pulleys. If you've got the cash, I would go with the PI parts with the LTs and mid pipe, then go from there.