1999 Civic LX Head Gasket

1. 1999 Honda Civic LX, 1.6L
2. Odometer: 225,000
3. Automatic Transmission
4. Sounds: A slight knocking/rubbing sound, which sounds like it’s coming from the left front wheel. I’ve had it checked by the garage that did my brakes recently, and been told that they cannot find anything.
5. Fluids: I have been using 5/30 full synthetic, with regular oil changes, but recently (past 6 months), I have had to add a quart every second fillup. Last oil change, I changed to 10/40 synthetic blend, and it has been at least 6 fillups and I have not had to add oil. But last week, my car started to overheat, I brought it in, the radiator fluid was very low, the thermostat was changed, and I was told to keep an eye the fluid from now on, that perhaps the head gasket will need to be changed at some point.
6. The car has been extremely reliable, and I have only done routine maintenance to this point, brakes, tires, muffler, timing belt, water pump, tune-ups, no major engine work.

I use this for a commuter car, it has mostly highway miles. I would like it to last me another year, and my question is this: Should I consider fixing the head gasket, or will this open up a can of worms, since this car has such high mileage? And if I do go ahead with the repair, how much should it approximately cost? I thank you in advance for your expertise and helpful advice!