I currently own a 2005 Lancer 1.6 GLX SR with CVT. The car has done about 106,000 km. With my heavy foot the car gives about 9.3-9.6 km/l with AC in the city. Since I'm in Islamabad this figure includes my trips to hilly areas that consume more fuel. On the motorway you can get up to 15km/l based on your driving style. In the winters I get 10.5-11.0 km/l but that is because I leave the car running for 10min in the morning and evening to get thenheater warmed up for use. Otherwise, it could go to 12km/l.
Based on your concern for fuel economy rather than parts I believe you'll be using cheap parts in the car and I would highly advise against it. Its parts are expensive but worth the extra money because you get an amazing drive and comfort level still unmatched by the 2017 Corolla and Civic. People will tell you that parts are not available but I will tell you that you can get all genuine parts from DEWAN Motors Karachi. You can always import from Dubai or Japan yourself.
Personally, I like to keep a standard inventory of two air filters, two oil filters and sometimes a set of spark plugs at all times. If you plan this way then you'll be good to go. I use only high octane because the car loves this fuel and drives very good on it. The new regular fuel of RON 92 will work but you'll feel the car protest every time you floor the gas. The car also seems to love Shell Helix Ultra 5W-30 fully synthetic engine oil.
Finding a well kept Lancer is a challenge because the ones who've messed it up are usually looking to sell it so you can see it going cheaply. The ones who've kept it very well can sell for much better. In my experience a well kept car of any brand sells very well without you having to advertise in the newspaper, on online sites, social media groups, etc. Log khud poocheinge ge ke "aap ne gari bechni tou nahin hai?" at petrol pumps, outside office, in social gatherings, etc. because everyone wants a well kept car.