Ok, if you find a 740iL with the sports package, I don't think you need to provide the car with a different suspension.
As far as engine upgrades, what did you have in mind? Swapping a V12 in there? Or did you mean a turbo, or supercharger? I'm confused about this engine upgrade?
I personally LOVE Dinan, I love thier exaust, and everything the offer. Each tuner is different and you have to figure out what YOU like. As far as exaust, the sounds and performance. There is UUC, Dinan, ACS, Remus, etc. Everybody complains about how much Dinan tuner's prices are. So they look elsewhere. Everybody is different and has different styles, tastes, and expenses....so that's what you have to figure out for yourself. If money isn't a factor, I suggest you buy a 2001 740iL Sport, then look for upgrades.