Probably due for plugs, oil change, filter, air filter, leads need replacing, transmission filter, fuel filter.
Oil and Oil Filter - Do this yourself $30-60 for oil (Fuchs, Motul, Castrol, Nulon etc.)$5-$10 for filter (Purolator or Genuine Holden is good). Get a new sump bolt and washer for $6.
Air Filter - Again do it yourself $20 for standard paper element - $80 - $120 for K&N reusable filter.
Spark Plugs- Anywhere from $20 for 6 to $120 for 6 - NGK, Bosch Super Pus , Denso plugs are good for standard, Iridium or Platinum if you dont wanna change them often and waste some cash. If confident do it yourself, otherwise ask mechanic.
Leads - Again easy to change. These are probably adding to your loss of economy. I just changed mine after 70thousand k's and i gained some power and economy back. Should haven changed long time ago. $50 - $120. Bosch, NGK , Eagle are all good. M.S.D meant to be the best but overpriced for what they are.
Check coils, but they should still be fine - Again could be adding to miss and fuel economy loss.
Transmission Filter - Get the mechanic to do this.
Fuel filter - Better for mechanics to do this as it can be a safety item. For some its simple.
Diff Oil - Mechanic to change. Often neglected by people including myself
Brakes and break fluid flush/bleed - Again get mechanic to do. Rotors might be due for a change.
Radiator/Coolant Flush - $40 Do this yourself if you have access to clean water during restrictions. Only use 50% Propylene Glycol(Safer for you and environment) or Ethelyne Glycol(Extremely poisonous-Standard easy to find) and 50% water.
Thermostat - $20 with gasket, again, very easy to change, do it will coolant change.
Radiator Cap and Radiator Hoses - Do it yourself $10 or so-Worth changing by now.
Regas and new fluid for Air Conditioner - Can be done yourself(fluid), better to ask mechanic.
Wiper arms and blades - Change yourself - Bosch have good arms and blades sets. $30 for 2. Top up washer bottle.
Drive Belt - Change yourself.Rubber or get a Gates performance belt.
Get suspensions checked over, maybe time for new shockers etc.
All the above, there is always info on here and other sites on how to do, some can do all, where as if its gont anything to do with petrol and making my car stop, i rather someone else do it proffessionaly.
If i missed something, please let me know. Doing the ones i told you yourself will save a lot of money, to may off the other stuff. Your sensors should still be fine at this stage, mechanic may wanna change vacuum/heater hoses.
Let me know how you go. Sometimes i go to mechanic like Ultra Tune for a check and they find all the faults, then i fix what i can and get prices for the other stuff and let mechanic do. THeres always someone on here that can do most things and help you.