Code reader for codes, if you didn't do plugs thats something that probably should have been done at 86k. Factory plugs say 100k maintenance... but you have no idea what the previous owner did. APP104s gapped to .054 and torqued to 15ft lbs for the plugs. Rebuild the COPs while you are at it, $30 for a boot/spring kit, don't waste money buying $400 worth of new coils.
Air filter, fuel filter and then fix whatever is throwing a code. You also will probably want to clean the MAF element so get a can of MAF cleaner (not carb cleaner MAF cleaner) and clean that as well.
If you have some sort of aftermarket MAF setup in there that could cause issues too, it did with my friend's 04.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.