To answer your question completely, there is no simple answer.
Here's the thing with supercharging, if you go positive displacement as in roots or twin screw, you can negate the install of the intake manifold. It saves you money, and you get great low end torque, but you are locked in to the engine profile of the runners that are part of the blower package. With a centrifugal you retain that flexibility at the cost of most of the low end torque gained by the positive displacement blower. I really don't consider that an issue because once you are in the powerband in first gear (properly geared), you never come out of it and you are more traction limited than torque limited in first gear anyway (even NA). The only place it would really matter is on the street - light to light, and if you are big into street racing this probably isn't the best forum to get most of your information from anyway.
Now as to the shop that supercharges for $3200....what kind of vortec are they offering? Is it used or new? My guess is that they are a vortec direct distributor so they can save on parts. However, if they are offering the smallest blower Vortec makes, non-intercooled, using a FMU to support fuel in this special, I would pass as it really offers no room for growth and will likely max out flowing no more air than can support 425RWHP.
The reason I recommend heads/cams/intake/exhaust first is this: If you do a blower first, and then decide to build a top end, you end up tearing off the blower to perform this. That completely negates the money you already spent installing it in the first place, now you get to pay extra for them to remove it, and to top it off you get to pay for a second installation. If you do it the other way, there is no backtrack installation involved....unless you decide to build your lower end at some point, which is not necessary for H/C/I/E or a base kit like you are asking about. But if you do both, then it becomes a necessity.
The Vortec supercharger option you mentioned will probably get you around 330-350 RWHP on a stock setup, and around the same amount of torque (maybe a little more or less). I say 350 because to go beyond that is beyond what a kit with a FMU can handle. Any more power without intercooling and you exceed the fuel capability of the system, which adds significantly to the cost of the kit in the form of injectors/pump and the installation associated. That power/torque level is going to be right at (or possibly less than) the levels that the H/C/I/E can achieve, with none of the other benefits the H/C/I/E can give you that I previously mentioned.
Even considering the special that your shop offers, unless I am just totally incorrect concerning the equipment they install, I would still build NA prior to going FI. I just don't see them offering the right equipment that would satisfy your needs for $3200.
Hope that answers your question, if not, let us know.