Hey, first off, welcome!!!
I am, like many folks, somewhat addicted to ebay... it's okay for some stuff, particularly hard to find european parts that we never got in N. America in quantity.
But, there is also no getting around the fact that ebay has its share of junk, whether used parts, non OEM stuff passed off as OEM, or aftermarket stuff that will ultimately produce less than satisfying results.
Also, ebay isn't necessarily cheaper than aftermarket dealers, or even BMW dealers... calling places like BMA, the euro depot, and even bavarian autosport (among many others) will often surprise you.
None of us are building like, matched-numbers concourse cars... but I would recommend the following:
- think about your goals for your car and try to clearly define your taste and direction
- look around at what other folks are doing --this is a start, not a finish; simply that we all have very similar cars and are trying to work out many of the same issues
- try to figure out what your car wants (sounds weird, but after a while, you'll start to see it)
- remember that nothing beats quality design, build and fit... on average, you get what you pay for.
When I got my car, it was hard to resist the urge to do the easy stuff... but the more I started focusing on learning more about the car, and doing real maintenance and real upgrades, ultimately, the more satisfying it became.
but hey man, it's your car, rock it your way... there isn't a single person who could/would stop you.
At the same time, the only person who can lower your standards is you... so try to come up with a 'vision' of what you want, check out all your available options, be patient, and do it!!
bottom line: learn as much as you can about the car, and do maintenance, or maintenance related upgrades before ebay mods!