If you're going to buy a helmet, think long term. They are expensive.
Yes, M helmets are OK for autocross, and some open track days, depending on host organization. SA helmets offer additional protections, as noted, and can have a smaller eye opening than a motorcycle helmet. If you think you'll need an SA helmet in the next 5-10 years, get one.
I'd also make the pitch for full-face, even in a closed car. If you roll, you'll want to protect your jawbone, too. More money at first, but you might be glad later. I've also heard "real" racers, who've been in fiery wrecks, make the same pitch based on fire protection, but that's only valid if you leave the face shield on.
Finally, fit. Different brands fit differently, and everybody's head is different. I haven't found a Bell to this day that fits my head properly, even though they make a fine helmet.
I bought a new-old-stock AGV M helmet off of E-bay for like 65 bucks shipped. Came with warranty and everything. I tried on some AGV helmets at a local shop.
