Hi Hass
First, welcome to the group and congrats on buying your 951, may you have a very long and happy ownership.
I think you'll find that most 951 owners have modified their cars in one form or another, so you'll find plenty of help here on this forum.
Your list is a good start although don't bother with the 'boost enhancer', it's ok as a first stand-alone mod but totally redundant once you fit a DPW wastegate.
I would advise beginning with the DPW, manual boost controller and new chips, personally I would choose a custom written chips rather than 'off the shelf' options. Get someone like Wayne Scolfield (Chipwizards) to tune it for you and you will see approx 270 to 290bhp at the crank and a similar amount of torque.
This will transform the car, you'll be able to redline in all gears, something not possible in a standard car. A word of warning, if your car is still on its original head gasket you may see this fail with the higher boost. If you decide to change the HG for safety, someone like Wayne would probably suggest this anyway, buy a 'Cometic' metal layer HG, this will withstand the extra boost and much more and thus will last for many years, if not for the life of the car depending on the tune.
At this level of modification I'd be inclined to fit a K&N filter in the original airbox rather than a new system, if you do more mods later and fit a MAF then yes you need to change the intake setup.
If you decide to fit one anyway, you'll find something suitable from Piper or K&N under their 'universal intakes. If you do buy one try to get one that takes it's air directly through the hole in the wing where the original airbox snorkel fits, the hole is large enough for a 3-inch tube. Avoid fitting a cone filter within the engine bay, you'll be sucking in hot air. We have all done it, my first intake mod was a large cone filter stuck in the middle, not a good idea, but hey, we all learn along the road, it did sound good though...
Good luck with your purchase, have fun, they are truly terrific cars...
Pete