Evo,
You are testing my memory as it was a few years ago when I had the thing, but if driven conservatively I would get just over 400kms to a tank (+/- 5%). This was 100% city driving though with minimal highway kms in that 400. Im afraid I cant help with an estimate on highway kms as the car is so unpractical for longer trips (unless you use the whole boot as a suitcase) it was never used outside of Sydney. If you were to use it agressively then that 400km estimate would fly out the window. I'm also not sure if would make any difference to range but my car had the old 16 bit ECU. Mazda updated to a newer 16 bit ECU (and im guessing would have resulted in a change to the mapping due to different turbochargers) in the Series 8. I would suggest that highway driving would provide significantly more range as you would be off boost more often. (well depending on how you drive on highways)
I didnt have any major mechanical problems with mine but was religious with servicing by a rotary professional and getting the oil changed every 5,000 kms / 6mths. I saw it as cheap insurance against blowing up an engine. My car had under 100,000kms so I cannot comment on the accuracy of claims that a rebuild is necessary between 100-150,000 kms. I didnt have any rattles from the door area, but due to the way the centre console trim (ie behind the shifter) is attached to the trans tunnel plastics, I did have looseness from this piece of trim. I do agree with T01-100's comment on the seat position though. I learnt to change my driving position, but more legroom & headroom would have been welcome. Something that can be fixed with your choice in aftermarket seats and rails I guess. Dont forget mine had a sunroof as well so my comments above are predicated on this fact.
They can have problems though with the sequential operation of the turbochargers as it involves a 'rats nest' of vacuum hoses under the manifold. This was addressed in the Series 8 somewhat (ie simplified) but I dont have the specific knowledge to tell you if it solved the problem with the secondary (ie larger) turbocharger either failing to operate or operating at reduced capacity. A common problem in the Series 6 and is one that increases with age/kms as the rubber vacuum hoses age through repeated heat cycles causing leaks. An easy fix is to switch to a big single!
Have you considered a S2000? Its sporty, has roughly the same amount of boot space and 1999-2000 models can be had for $30K. Much much easier on the fuel too. Not too much chop though.
Although I have moved on from the rotor world I will never forget the wail punctuated by the shift chime so unique to the 3rd Gen RX7. Its an absolutely timeless shape and if you can live with its eccentricites is a great car to own and drive. I still dream of owning a Series 8 RS+20B combo.