I have the Dinan stage 2 suspension on my 02 M3, with front and rear strut braces, as well as the Dinan cold air intake and software. I also have the UUC mid pipe, SuperSprint exhaust, drilled "bmw euro" rotors with upgraded pads, and a ACS style carbon fiber front spoiler/splitter.
You are certainly safe with Dinan with the whole warranty thing, but others certainly make good products. The only issue I have had is getting the UUC mid pipe to fit properly without hitting the undercarriage.
Have fun, but be aware of that these items will make your car ride firmer and be louder. Not a bad thing if you are into it, but if it is a day to day driver, and you like the relative comfort of the OEM M3, these updgrades will tilt you more towards the "sporty" ride and feel. I do not drive mine every day, but when I do, the thrill is elevated due to the upgrdes I have made. The upgrades also motivate you to go to the track more often, as the enhanced performance level far exceeds what you can responsibly tap on the streets.
Good Luck