Is my old track car actually safe enough with the original seat and belts?

Ran a mostly original E30 for about five seasons of track days, so I'll tell you what I actually did and what I skipped. The guy in the paddock is half right. Stock lap belts on a stock seat are genuinely poor in anything serious because you submarine under them, but the fix isn't necessarily a harness. A harness bolted to a stock E30 seat is a mixed bag. The seat back will flex and fold rearward in a hard hit, and once it does the harness geometry goes to pieces. Worse, a harness keeps you strapped upright where a street belt woud let you move with the car. So a harness on that seat is mostly wasted money unless you also address the seat back. The middle ground that worked for me was a proper seat mounted to the factory rails or proper brackets, and keeping the regular belt. That alone transformed how planted I felt and how the belt sat on me. If you go that route, mount the seat so the belt anchors still work as designed, or add a harness bar behind the seats and run a 4 or 5 point with the correct angles. Do not mount harnesses to the floor or seat rails on thier own, that's how people end up with belt angles that don't hold them down at all.

On the cage, I went bolt-in and I don't regret it, but the paddock guys have a point too. A cage with a stock interior, no helmet, and foam padding that's older than the car can absolutely put hard metal closer to your skull. If you do add one, get proper padding rated for roll structures on every tube near your head, and think about wether you'll actually wear a helmet every session. If the answer is no, a cage adds less than you'd hope. My order of changes was seat first, then belts and harness bar, then padding and the cage last. Left the interior trim alone the whole time. Car still looks like an old BMW inside, it just holds me in place properly now. Biggest thing I'd say is don't let the paddock talk spiral into a full race build before you've done the two cheap things that matter most, because those two alone cover most of the realistic scenarios at track day speeds.

Same expirience here, can vouch for the seat-first approach. I put a secondhand fixed-back in my old coupe on stock rails and the difference was immediate, mostly because the factory driver's seat foam had gone flat after 30 years and I was sitting way lower than I realized. Belt finally sat across my hips instead of my stomach. I left everthing else stock for two more seasons after that and never felt underdone out there. Only thing I'd add is check the seat mounts torque up regularly, mine worked loose after the first couple of weekends.

One thing I'd add is the belts themselves. My 87 had original webbing that was faded and stiff, and one retractor barely locked anymore. Swapped in fresh stock style belts before anything else and it was cheap peace of mind. 30+ year old webbing and locking mechanisms aren't something I'd trust at speed, even if everything else stays original. Takes an afternoon to do both sides too.

Went the single roll bar with a harness bar route in my own old coupe instead of a full cage. Cheaper, keeps most of the cabin untouched, and gave me somewhere proper to anchor the belts. I wear a lid every session no exceptions, which made that decision easy. What I left alone was everything cosmetic, dash and door cards and all. Only other thing I did was check the seat mounting bolts for the first time in decades and found two finger tight, so whatever seat you end up in, actually torque everything down and recheck it after a few weekends.

This is all useful stuff, my old car is from the same era and I've been putting off thinking about any of this. One thing I keep wondering about is the driver position part someone brought up, the sitting-too-low thing. How do you even tell if your seat has sagged over the years? I feel fine in mine day to day but I've never driven anything to compare it against. Also does swapping to a diffrent seat make it harder to reach the handbrake and shifter on these old cars, or do you get used to it straight away?

Update: pulled the driver's seat out at the weekend and the foam is completley flattened, exactly like the second poster said. No wonder I was sitting so low. Belt sits across my lap now instead of my hips. Before I start hunting for a fixed-back, did anyone find one that bolts to the stock rails without adapters? Not keen on fabricating brackets myself.