Are there specific parts that really need to be OEM? I know that it's probably preferable for all of them but are there some that absolutely need to be OEM?

 

Well it depends on what you mean by OEM. OEM generally means the original end manufacturer. To some that is "genuine" parts only. To most it means the company that produced the part for the vehicle maker. Such and Lemforder or Denso. So when I buy parts, I do my best to fund out who made the part for Rover and I buy that brand form a reliable source. Now if that part is seemingly "expensive" and a less expensive one can be found from a reliable brand, I will consider it. Such as the LR3. The factory water pump is well-built, but others have gone with the AC Delco water pump simply because its very inexpensive compared to OEM and it seems to be robust enough. But for a MAF sensor, I would only go OEM which is Denso. So the way I look at it, for sensors and suspension I generally go OEM. For belts, hoses I just go with a good brand. For some items, I scope out the forums for alternatives that seem just as good as OEM for less. But to answer your question directly, nothing MUST be OEM. Just realize there are many inferior products out there and while you may save money upfront it can cost you in the long run. Also, never buy ÜRO brand... ever...