The mechanic said that the valve seat problem has a low chance of happening if I make sure the engine temperature is being regulated. He said he's seen SPI's with 200k+ miles. Depends on how you drive it. I know there's some truth to that, but I also know tomorrow could still be the last time it runs. He talked me out of doing the cylinder head replacement, which he said would have been upwards of $2,000 alone (seems a bit high?). He said the rear shimmy was happening because of a bad wheel bearing and a bad arm. He is going to replace those and do a 4-wheel alignment for about $400. He said the engine rattle was from 3 bad engine mounts, which he is going to give me a quote for, but said wasn't necessary right now.
This guy only had 5 reviews (all 5-star) on cartalk.com, but everyone else in my area had just 1 or 2 reviews.
My plan (assuming the rear shimmy issue is actually fixed when I get it back) is to just drive it and keep my fingers crossed. If the valve seat drops, then I'll consider a Zetec swap or just junking it and looking for something else.