Hello and welcome to the site. There are a few other members around the Warwickshire /Leicestershire borders area, so if you need any help you are well placed.
For the parts you mention; OEM suspension is Sachs and readily available from Eurocar Parts or Car Parts for less. For alternatives, plenty of people have fitted lowered suspension so just ask on here for recommendations.
You correctly identify bushes as probably needing to be looked at and opinions are divided whether to fit polybushes or stay OEM. I think we all agree that replacing the diff carrier bush with a Powerflex polybush is the best option, as it is simpler to fit and doesn't have the same problems as the OEM. Likewise for the front wish bone rear bush, preference seems to be for polybush. After that opinions seems divided.
Due to the age of our cars many rubber parts are deteriorating so check, ait intake hoses for splits and the rubber drive belts as well as the water system.
For engine sensors people generally have found OEM from dealers to be more reliable than aftermarket.
Sopers of Lincoln offer 10% discount on parts and if you ask othe dealers will also give a discount.
Hope you enjoy your Z3, it certainly looks a nice example.