The 323 and 328 weren't that much different in terms of power - at least not in the way the 325 and 330 are different.
Now, the one thing to find out is if the dealer car is a CPO. If it is, then I suggest going for that one. You will not know how valuable having the CPO until you need it.
The xenons can be retrofitted so that's not much of a concern. Many people have said the HK system isn't all that but I've always felt it was more then adequate for my needs.
The only unfortunate part is that it does not have the Sport package. I've always felt the seats alone were worth the option. The regular seats really suck in my opinion. They are flat bottomed, little to no lateral support, and just aren't as good as the sport seats overall.
So here's my suggestion: If the 328 has CPO, then get that. If not, then I say get the 323 since it's optioned out better and just mod it up with a nice supercharger if you really feel the power isn't adequate.
