Re: 2000 Taurus Misfire
If you're comfortable with replacing an engine, then I'd replace the head gaskets. As you mentioned it may have overheated, so the heads may be warped. The gaskets should run around $40-50 and can be purchased online (Jegs, Napa Online, etc). While the heads are off you can get them checked for leaks and proper balance. If you can do the labour and have the time it'd be more than worth while in my book.
Worst case, is that the piston rigs are gone, or the rings are wearing down parts of the cylinder wall causing blow-by; or low compression. Also, with the heads off, check the valves for any built up carbon, and clean as necessary.
I'm not sure about this "leak down" thing you mentioned, unless this is a new term that I haven't heard yet to replace an old term of some kind. Anyway, keep an eye on all fluid levels. If you don't mind spending $20-25 or so, you can send in an ounce of your engine oil to a lab for diagnosis. They'll tell you if there's coolant or other crap in there that shouldn't be. I get diags done on my vehicles every year, half way between an oil change.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!