congratulations and welcome.. For tires, I like BFG Comp 2's.. but they are a 340 wear rating and are not for someone looking ot get 40,000 miles out of them. Nitto's are great but imo not for all season or in the rain.
I'd vote to keep the car as stock as possible. It's a slippery slope once you start doing mods.. Here is what I would do if it were my car
Maximum Motorsports subframe connectors- these cars twist like swiss cheese and yours being a convertable will rattle and squeak over every major bump or crack, These will tighten up your car and greatly improve the rigidity
130 amp 3G alternator- your stock 65 amp alternator is barely adequate for a stock mustang with no aftermarket stereo, or E fan. Plus the wiring is a known fire hazard
MGW or PRO 5.0 Shifter- this will eliminate the slop in your stock shifter and provide more positive shifts . They also have stops so you dont overhift and bend your shift forks
K&N Air filter- this will give you a little increase in hp and is reusable
3.73 rear end gears- this is going to be the best bang for the buck performance improvement. It will make the car feel totally different and give more out of the hole.
All of these with the exception of the subframe connectors are bolt in and reverseable should you decide to put the car back to 100% stock. Keep the old parts if you decide to do so.
I would also spend the $255 and get the Maximum Motorsports break upgrade kit. The stock Mustang brakes are terrible and this kit has better pads and shoes, Stainless steel
brake hoses to replace teh 30 year od rubber ones, and stainless steel caliper bushings. This will provide you with greatly improved brakes on a budget.
If you really get the HP bug I would recommend a base V3 kit that is completely bolt on and you dont have to change much except your fuel pump. You'll see around 75 hp increase for about $3000.