your plan is good but alittle off target.
improved intake/exhaust is good and the mild cam also for a turbo setup.
the stick trans, 1500-3500 range and decent mileage are not good.
now to answer some of your questions:
1. a turbo will most likely decrease mileage.
2. cost of around 2000.00 all done with a carb. realistic
3.30-50% increase mainly in torq because of your low rpm level. a 250 will run much better in the 3000-6000 range for power. in this range the hp will come up sharply. and still provide good economy in the 1500-2500 range.
tom
