[Three Peaks]

From your description of your wants and needs, the only choice I can see would be the Factory Five. From a base kit with some add-on options, you can modify to your hearts content much easier than the others mentioned IMO.
From IRS rear for autocross and track work, to modifying the seating arrangement, to double roll bars, to any engine you want to install, adjustable suspension, multiple brake options, as many options for your dash, console, etc.. as you can imagine, and soft and hard tops available.
And there are many FF5 builders who do great work and know these cars and what it takes to make them reliable, safe, and fast- or whatever you are looking to accomplish.

Just my 2 cent's worth- and I don't even own a Factory Five---

Bob