Warning lights and gas consumption abnormally High

Bought vehicle second hand from original owner who took immaculate care of the car and always serviced at the dealership. Worked well for the first couple of years, then the VSC, 4WD and check engine lights came on. Mechanic said needed to replace plugs and coils, which was completed at a cost of over $2,000.00. Drove almost four months and the same three lights came on again, and the vehicle felt like it was going to shut off, especially when I stopped at a light. Took to a Toyota dealership and they said one of the plus/coils needed to be replaced for $650.00 however, this did not fix the problem and now the dealership is saying need all of them fixed. I am a lady that knows little about cars, I am at the mercy of the mechanics, and it troubles me to know that the dealership diagnostic check could not pinpoint and solve the problem, especially as we thought the replacement of all coils/tubes a few months ago, at an extremely high cost did the trick. As you can appreciate, I paid top dollar for a twelve year old vehicle in excellent condition and have now spent over $3,000.00 more in a couple of years, which I will never be able to recoup if sold today. Also, the fuel consumption is high, I could fill the tank and after driving about 20 kms, literally see the tank gauge indicator move significantly, i.E about 50 kms. And a quarter of a tank later. Can anyone help me solve this recurring problem at a reasonable price so I can still utilize the vehicle for several years? Everyone thinks it is new the condition, way it looks and low kms. I am not sure but could it be that the first time we replaced all the coils/tubes they were not Toyota parts as they were very expensive. Any helpful tips. Comments, suggestions, would be greatly appreciated. I am really distressed about this situation as the repairs are too costly, and I bought used at a high price, thinking I should get a lot more mileage out of the vehicle. Everyone told me Toyota's and Honda's never die, at least not at the low mileage of this one. Thanks.