Heh. It occurred to me during the two-week window one has after configuring one's car that I hadn't test driven one. So after my sales guy made an appt for me at the closest showroom, off I went. The showroom is a useful place in which to see all paint colors, wood options, leather colors and so forth. The demo vehicle you are assigned may or may not have the options in which you are interested (although you can always ask in advance to improve your odds).
So back to your situation. Most of the M3 options will be similar to what's available now in an MS. You could call a showroom sometime in the next year or so, explain that you're an M3 reservation holder, and off you go.
The most costly options (P, assuming there's going to be a P), a larger battery pack, and AWD are all fairly self explanatory. Trim you can see in a showroom or online.
The audio packages, well... this depends upon you. I went middle of the road with the base package with the intention of Reusifying it. If you don't know a FLAC file from a turnip, just go with the base package and call it a day. If you've got $20K or more into your home entertainment, get the upgraded audio and then call Cliff at Reus for an appointment or to schedule an install at the next TMC Connect soirée.
Lastly, the irony is if you are not an owner and not in CA and did not sign up on day 1, then about your only shot at the full tax credit is to order a fully-loaded M3. Which somewhat obviates the point. Or not, depending upon how you look at it. Some look at the tax credit as free money and others need it to make the car attainable in the first place. Both cases are valid as, contrary to the belief of some, the point of this tax credit is not to help the less economically endowed. The point in this case is to accelerate EV adoption. Doesn't mean people making, for example, $40K/year can't benefit from the tax credit in order to get an EV. It means people making $40K/year will drive Fiat 500es. Nothing wrong with that. Tons of people in the San Joaquin Valley do just that. Why? Because they get $13K back to do so instead of $10K or less. In time, when used Model S and used Model 3s are $20K, they'll make great first cars for everybody - safe and no gas costs.
Meanwhile, don't worry about the options. But do get the black headliner. The light-colored textile one (no longer offered) was horrible. This and thousands of opinions just like it are all right here and at TM for the reading. And you've got a whole year and a half yet
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