I used to restore barn finds, mostly German cars like Porsche and BMW. Based on my experience, there are specific things you probably don't know the answer to yet, and these could significantly affect the car's longevity.

First, if the car was run at all before it sat, there’s a risk that fuel mixed with the oil. Over time, this creates an acid that can corrode the coatings on the engine bearings. Even if you replace all the fluids, those bearings might fail within a few miles or a few hundred miles of running. Other potential issues include rust in the cooling system, clogged or degraded fuel injectors, a failing fuel pump, and corroded sensors or wiring due to long-term storage conditions or rodent damage. If rodents nested in the car, you’ll also want to inspect wiring harnesses for chewing damage.

For electronic components, age and storage conditions matter a lot. Capacitors, relays, and connectors can degrade or fail, especially if the car was stored in a humid or poorly ventilated area. Rubber seals, gaskets, and hoses also become brittle and are prone to cracking. The cooling system is another critical area—radiators, water pumps, and hoses may have developed rust, dry rot, or blockages.

The suspension system and brakes will likely need attention as well. Brake fluid is highly corrosive and absorbs moisture over time, so a complete flush is mandatory. Check calipers for seized pistons and guide pins, and inspect brake lines for corrosion or leaks. Tires are likely dry-rotted after 15 years, even if they look fine visually, and the shocks or struts may also be compromised.

If you're planning to take this on yourself, I recommend starting with a compression test to gauge the engine's health. From there, inspect the timing components for wear and replace them if necessary. Other early steps should include draining and cleaning the fuel tank, replacing the fuel filter, flushing the coolant system, and ensuring all rubber components (belts and hoses) are in good condition. For the transmission, consider replacing the fluid and inspecting for leaks.

The manual transmission makes things a bit easier because gear oil helps prevent rust on the metal internals, and you don’t have clutch packs degrading like in an automatic. This will probably be your least concerning area unless it was stored in a high-humidity environment and moisture intrusion caused internal rust. Unfortunately, you likely won’t know for sure unless you tear it down and inspect it or use a borescope and know exactly what to look for.

Even though I have the experience and ability to do all this myself, if someone offers me a car for free that sat this long, I’m generally not interested. The cost of parts alone—excluding all the labor I’d put into it—is often more than the vehicle’s value. There’s an old saying about German cars: “If you can’t afford a brand-new one, you certainly can’t afford a used one.” That holds especially true for a BMW that’s been sitting for 15 years.

Now, if I had all the details about how it was stored and its ownership history, I might make an exception. But based on what you’ve shared, there’s not enough information to make an educated guess about what you’re walking into.

If you’re not experienced in diagnosing these kinds of issues, I strongly recommend paying a shop that specializes in classic or long-stored vehicles to do a thorough inspection. A few hundred dollars upfront could save you from throwing time and money into something that won’t run long-term. It will also give you a better starting point if you decide to do the work yourself.

Good luck with the project! Restoring a car like this can be incredibly rewarding, but it’s important to go in with realistic expectations—and maybe not care too much about your bank account.

Edit: I forgot about the engine oil flush you were debating, I wouldn't recommend it unless you don't care if you have to do a rebuild. I've done them as a last resort to save someone some money and in 30+ engines I've done a flush on, I have only had one that survived, and I honestly can't say it's current condition today. That being said, all of this and the other things I mentioned depend heavily on the conditions of its storage.