The thing that is unclear to me are how many miles do you and your wife plan on putting on your cars total and how do you split those miles amongst the (if I remember correctly) 4 cars?
I'm only seeing great condition, no paintwork, low mileage and owners E46 and E36 M3s really go up in value in the near future. There are still just too many of them around that ones that aren't investment grade or close to that are staying flat. If you need your 330i or the replacement to be a daily commuter or otherwise a car that you can add a lot of miles to, I don't think you'll really be able to ride that appreciation train up, barring the unlikely situation of a market-wide price run up similar to air-cooled 911s in the past few years.
A few more things to think about:
- E36 interiors are pretty dated, and the E46 interior is a serious step up. The E36 interior is a big knock against the car, at least personally for me. I don't know if you like or care about that at all.
- How important is the automatic transmission for you? It seems the E36 M3s that go up in value are manuals. The E46 M3 SMGs can be clunky and troublesome.
- As with any used car, you'll likely have to put in a little bit of money soon after acquisition to completely sort out.
- I expect the M3 maintenance costs to be higher than the 330i, especially the case with the E46 M3.
- It looks like in Colorado, when you purchase a used car, you owe sales tax on purchase price. That's additional cost that you'll incur changing to a M3.
- The 330i has depreciated quite a bit already, and should be at the point where marginal depreciation should be getting less and less. Even in the extreme case of driving it into the ground (only scrap metal value left), how much money will you lose? That is the max potential downside of keeping the 330i to trade against the added costs and, as well, potential appreciation of the M3s.
- We've had a good run of economic growth for the past few years. There's already talk that indicators of a slowing economy are cropping up. Add in trade wars and other global uncertainty, how much longer will this economic growth continue? When will the next recession hit? I don't see E36 and E46 M3s being enough of a blue chip yet to be recession-proof.
I would only switch to a M3 if you are itching for something different to drive or play around with, or if the 330i doesn't already meet your needs and the M3s will. I'm not seeing the business case with the costs of finding, buying, getting up to snuff and maintaining a M3 over your 330i. I only see the M3 appreciation offsetting that if you buy at least a close-to investment grade vehicle to start and put very little miles on it over time, and hold the asset for a few years.