When I bought mine, I didn't know what was recently done to it, and neither did the previous owner (or so he claimed, I'm still trying to figure that out ). But anyways, I decided to do plugs, wires, cap/rotor, pcv valve, fuel filter (a must), cleaned the K&N filter, new radiator cap, cleaned the MAF and IAT sensors, and changed the oil. The only things I haven't got around to yet are the tranny fluid and rear end oil changes. I'll get those in the spring after she sleeps the winter off. Some of those things might have been done shortly before I bought it, but its piece of mind for me knowing that everything was freshly changed so I could start off on a clean slate.
Car has had routine oil changes and was a DD all of its life. My friend bought it from them and I bought it from him. Its got a new KC clutch, 4:10's and steeda sport shifter. Its got 150k on it but for a never been wrecked mustang 3 grand wasn't a bad deal.
I'd add to the rad cap idea - coolant and a new hi-po t-stat (hoses too if needed, esp the PITA small hoses on the pump). Also check things like U-joints, brake linings, and the like.
Congrats on your purchase. I just grabbed a mildly modded 95 GT off a local car dealer who didnt know what he had. Yea, the clock says 139K, but the car is tight and runs like a banshee with its azz on fire. I paid a bit more than you though .. $4250 was drained from my bank account ... but worth every penny.
Following what has already been posted, just start at the front of the car and work your way back. Check everything and anything that could be worn and need replacing or cleaning.