Ryan,
I too am in the early stages of this lovely insanity, but I have done a bit more research since I've been thinking about it for a while. Hopefully I can offer you some benefit from what I've found. BTW, I started looking at Cobras before switching to Speedsters, so I can do a decent comparison.
First, I assume your loose specifications (350hp, 2100lbs, IRS) are based on Cobras. These are not entirely applicable to Porsche Speedsters or Spyders. First, all Porsche replicas have IRS, although some are of the swing-axle variety. Second, you'd have to add considerable ballast to get a Porsche replica (Speedster or Spyder) to weigh anywhere near 2100 lbs. Most will be in the 1600-1900 range. And finally, 350 horsepower in one of these would be difficult to achieve and absolute insanity to drive. Most of these cars seem to run in the 100-170hp range, with a few falling on either side of that. With so little weight to move, 100+ hp will be plenty entertaining. It will also be a lot less expensive to maintain and insure.
The main reasons I switched to Speedsters from Cobras were:
1. A nicely done new Speedster can be had for around $26k. Despite what some of the Cobra vendors say it would be difficult to have one of equal quality for less than $35-$40k. Nice used Speedsters can be found in the teens.
2. Insurance and gas will both be more expensive for the Cobra. Some insurors won't touch them any more. Speedsters, on the other hand, seem to be no problem.
3. The Speedster guys are a blast! I had a great time with them at the Carlisle show last month. There seems to be no snobbishness or testosterone overload like I detected on the Cobra site.
Now for some hopefully more useful info. You say that you and your father are not particularly knowledgeable when it comes to building cars. For prices in the mid $20k range you can get a new Speedster delivered from several manufacturers. If you really want to built it yourself there are also some options. You can go with a kit from Street Beasts, Thunder Ranch, Beck (Special Edition), or JPS. The latter two (I don't know about Street Beasts) will be glad to work with you on partial builds. I know that Carey Hinds at Special Edition has two option packages: a "roller" (all but engine installed), and a built up kit that includes paint but requires you to do the drive train and some other components. I believe this last package includes the installed interior, but I'm not sure.
There are a number of folks on this site that I've learned a great deal from. It is definitely the place to start. You and your dad have a lot of fun ahead of you.
Lane Anderson