Well I used to have a 94 GT so I have a special place in my heart for them. I personally like the body lines on them. As for the route you should go with your newly acquired one,
@Davedacarpainter gave a lot of great advice. I don't care for convertibles but you can make it respectable. Like everybody else said, SFC first. You NEED them because verts flex a lot more than cars with roofs.
Suspension would be next. UPR would be the best bang for the buck and they are proven to work. UCAs, LCAs, shocks, struts, k-member kit, bumpsteer kit, caster/
camber plates, etc. As for the engine, I would suggest a H/C/I package like Trickflow. I prefer AFR but like Dave said AFR doesn't offer a complete package like Trickflow and Edelbrock does.
https://lmr.com/item/TFS-K514360350/1987-93-Mustang-50L-Trick-Flow-Track-Heat-Top-End-Kit-Silver
You'll just have to add a few other things when you get that like an intake elbow(its only for 94-95 5.0s), 70mm TB, 24# injectors and bigger MAF meter calibrated for the injectors, bigger radiator, full exhaust(headers, mid pipe, and mufflers), TKO 500 transmission, nice clutch, and 3.73 gear. If you have any $$ left over upgrade the front brakes to the 13" Cobra brakes. You don't need to upgrade your electric fan in that car mine worked fine when I had mine. All this will probably eat up that $10k budget especially if you get a shop to do all the work. As a matter of fact, if you get a shop to do everything cancel that TKO 500 I put in the list. Just get a nice shifter for your T5(Steeda Tri-ax, Pro 5.0, or MGW). Oh yeah, and a get it tuned on a dyno to get every bit of HP you can get out of the combo. And after all that you'll need sticky tires.
Oh yeah, forgot to mention bigger fuel pump(255lph or 340lph) and an adjustable fuel pressure regulator.