Ford LS focus 2007 advice

I bought a Ford LS focus 2007 manual from a dealership (in Australia btw) multiple red flags I ignored but I was desperate a car and bought it as it's my first car i don't know much. Essentially the dealership put normal water, like tap water in the radiator and the mechanic replaced it with the proper fluid needed. It drove great at first 3 months in I get a check engine light go to a mechanic and it's a swirl flap issue code (EML P2008 CODE SWIRL FLAP AIR SHUT OFF SOLENOID) they say itll be costly to fix as the intake manifold needs to be taken apart to fix the issue. They say I can still drive the car and it's ok, I switch the type of fuel I was using (initially was only using 98 premium) now to 95. Engine light turned off and for a month it's been driving great, recently the wiper fluid hasn't been coming out so I opened the hood to see if I needa add water and saw my radiator is full of black sludge, I think it's oil and I have a blown head gasket, does anyone know how much it'll roughly cost to fix and wether it's even worth fixing. I bought the car for 3999 and if the costs are more than the car is worth should I even do anything. Additionally I've had issues with the dealership and I'm filing a vcat (Australian consumer affairs) report as they never transferred the warranty to my name and I don't have coverage yet the car is in my name and I have insurance so just a bit lost.