Congratulations on your purchase matey. There are a couple of good places to acquire information bout the fc3s eg. rx7club.com and nopistons.com Check these places out for general information. With regards to looking after your engine, pretty much how you look after pistons. SOme points to follow.
1) Regular engine oil change
2) Change out fluids (probably never done by previous owner)
a.Diff
b.Tranny
c.Radiator (use only distilled water,coolant and Redline Water Wetter)
3) Considering the main problem with rotary is cooling i'd suggest you keep a close eye that.
4) Some gauges would help. Unlike ricers, the rotary can definitely do with gauges especially;
a) Boost (if you're running turbo)
b) Water Temp
c) Oil Temp
d) Oil Pressure
The list just goes on i suppose.
Mileage really depends on the condition of your engine. On the FD i get approx 200km from RM50. Still not tuned yet so hopefully with some tuning i'd see some difference.
Are you going to join us for the Frasier's Hill Drive? You should come and meet with other RE owners to learn more bout your car and ways you can mantain her. Keep us posted. And before i forget - Welcome To The Club Mate!!