You might look through Facebook Marketplace, Craigslist, or your local dealerships and notice you can find a nice-looking BMW N54 335i for a low price. Now at such low-price levels, you must keep in mind that they require a lot of attention. But be warned, there is an 85% chance you will be buying a dud of a car. Simply, because the prior owner did not follow a maintenance schedule.
Back in the mid-2000's when the E9X's released. BMW had free oil changes and helped maintained the car until 50k miles. They even considered some fluids lifetime fluids. I came to a conclusion, the word lifetime is a play on words. This means the lifetime of the car, which is about 100k miles. So by that mileage, most all your fluids, bushings, and accessories should be changed.
The first owner takes the hit on the car, paying around 35k for the car, and generally trades it in around 50k after the CPO or BMW warranty is up. The mod scene isn't well established yet, the aftermarket parts price is still high, and you can still consider it a foreign.
The second owner pays about 25k and enjoys the car until about 100k when they notice issues here and there. Such as leaking oil filter housing and even ratline wastegates. The most that have been done in maintenance are oil changes and probably some light mods, such as a Procede piggyback and some AR Design downpipes.
Now the third owner gets a chance at the mighty BMW N54 335i, pays about 15k, and inherits years of neglect and possibly a sagging headliner. They tried to revive the car and dropped thousands on this car to bring it back to life. Such as new turbos, changing all the leaking gaskets, and after awhile, they give up since they are hemorrhaging money. After a year, they will finally say I'm done with BMW's and the car gets kicked on to the next owner.
Anyone after the third owner more than likely paid in the 3k to 8k range. The car is far from a foreign now and a full-blown citizen doing jury duty. At this point, the entry price is quite low, and the first thing you should do is get the maintenance up to date. Don't just throw power mods, and they complain later about how bad this car is. For example, this BMW 335XI that @el_nOshi worked on View Video.
If the prior owner changed the oil filter and cooler gasket, the entire cooling system and components could have been saved. This encompassed oil mixing into the coolant, and oil started to leak all over the engine bay. The first thing it hits is your alternator, accessory belt, Pulleys, and tensioner. The life of those items has been shortened or deteriorated. Also, the oily coolant gets pushed into your water pump and thermostat. Leaving you with hours of flushing out the whole engine.
So with the oil mixing with the coolant in other platforms, that's a red flag stating the head gasket is blown and other significant problems possibly. On the e9x series, a blown head gasket is not likely at all. Most E9x series you see for sale on Craigslist and such that state head-gasket it's typically just a 15 dollar part that needs changing, and then a bunch of dollars to replace all the damaged parts, but as long as the body and interior is clean the car can come back from the brink of scrap yard to on the road again.
After all this neglect, the owner will realize they have a repair bill in the thousands. They then trade the car in, and when the dealer looks at the car, they push it on to the auction where another dealer pays for it and usually ends up at a buy here pay here type of establishment. They put it through a good cleaning and get it to a point where a test drive is feasible. You then see the ad and think, what a steal of a price! Run like the wind; these cars aren't for the everyday consumer. You will need to invest a good amount of money to have it in running order.
If you see an n54 powered car such as a BMW 135i, 335i, 535i, or x35i priced too low, my suggestion is. Would you please exercise diligence and get a PPI at an indy shop that is familiar with modified n54's? I have seen BMW dealerships that have given good ratings on cars that didn't deserve it. Also, if you can work on these cars, this is somewhat of a come-up. You can haggle a lower price and put in the work and fix it, and if you want, flip it for a profit.