NC0110100

1997 Legacy Outback had occasional hesitation when feathering accelerator. Now idles rough and seems to miss at low speed. Unusual fuel or exhaust type smell just at starting (not like antifreeze smell to me). Highway speed runs fine. Check engine light comes on. Has given codes for cylinders 1-3-4 misfire so plugs and wires changed. Also code for mass air. Recently put in Amsoil air filter but put original back in to eliminate that issue. Dealer wishes for me to drive it 2-3 weeks so they can check plugs to eliminate head or gasket issues so as not to unnecessarily replace mass air part at $400. Any advice?