Add running and insurance and storage costs for ~17yrs and I'm pretty sure you're breaking even at best!
I meant to actually come back to this and add that only the 333i is probably in an 'investor grade' category. The others will have needed a lot of maintenance/running costs and would also have depreciated further through use.
The iS might seem like one given the asking prices, but when I see guys spending 300-500K (over and above what they are spending on the base car) for 325iS restorations that actually aren't all that great then I also would question whether the iS is investment grade as well. I think if you managed to own it from late 90s til now and kept it mint then you're smilling, but not sure how many of those guys there really are since most seem in dire need of restoration or have received the equivalent of Simon Cowell's plastic surgery. The genuine minters that come up get snapped up at whatever the asking price is (driving this belief that you can take a 250K car, put 200K into it and ask R750K+... doesn't work that way).
It has never been a cheap car to buy or to own and neither is it a cheap car to restore. There was a Carte Blanche episode about these and 200K doesn't buy you much of anything - especially when the starting point is a shitbox like some of the cars in that segment. Used parts and best-effort service. I understand good restoration work is expensive, but then the point is at those asking prices (even if they are ever reached) there isn't exactly 'profit' in there.
The E36 is really nowhere and I don't think that it will get very far because you have the E46 and E92 on the other end that are probably better for both nostalgia-buyers and these elusive collectors we hear about that are just dying to pick up cars decades later at the peak of their asking prices and with huge mileages/lots of hidden issues.