Fastener tensile strength is designated by the numbers marked on the bolt head and nut. Most people are familiar with grade 8.8 bolts. The first number designates it's tensile strength. In this case the 8 means the UTS (Ultimate Tensile Stress) is 80Newtons (or approx 80kgs) per square mm of the bolts cross sectional area. The second number 8 tells you that it's yield point, (the point at which the bolt starts to stretch) is at 80% of the UTS.
8.8 bolts are sufficient for most parts on a car except for very high cyclic temperature critical such as cylinder head bolts. On a Z3 these are 10.9 grade.
For fasteners where corrosion is more of a concern than tensile strength you could select galvanised or zinc plated ones. Plated or galvanised fasteners wont give 100% assurance though because these can become susceptible to chemical imbalance when they come in contact with salt which we get when our roads are gritted etc.
316 stainless has a UTS in the region of 50N/square mm.
A4-80 grade stainless fasteners will give you similar properties to 8.8 steel fasteners but they are usually available by special order subject to order quantities etc
Hope this little insight helps, but as a rule of thumb, go with what the manufacturer fits in terms of grades and types of steel etc. Bolts are available with different types of head such as hexagon, allen, torx etc and these can be chosen to suit yourself or for personal preference and wouldn't have a impact on their suitability from an "engineering" point of view
