Have a look at the SLE models as well. Same styling as the SSEI with all premium goodies but without the S/C.
Avoid the GXP model like the plague. The Northstar V8 was troublesome with high repair costs.
You may also want to look into the Oldsmobile Aurora - same platform as the Bonneville with a bit more premium interior.
The SLE is close, but it is not an SSEi, some SSEi standard features are not even available on the SLE. The handling/ride/suspension is much more aggressive on the SSEi as well. The SC is the biggest appeal to many about the SSEi, it is fun when you mash the go pedal and still gets great mileage if you lay off the throttle.
As for the GXP, I would strongly disagree with you about that, I have owned most of the most popular Bonnevilles and there is nothing like the sound of the V8 in my GXP. The 2004 & 2005 Northstars are quite reliable (relatively speaking) and generally are not plagued by the earlier N* issues. I would agree that if the N* breaks, get that 2nd mortgage to fix it, you are gonna need it. The GXP has many unique features that you cannot get on any other model, carbon fiber interior trim, suede seat inserts, V8 power, huge 4 piston front disk breaks, etc, etc. The late model Bonnevilles are not what I would call low maintenance cars, no matter which trim, but they are large, comfortable, fun cars to drive. They were never meant to be the equivalent of a Honda/Nissan/Toyota grocery getter.