Randall said it well, the difference between the TR4A, TR250 and TR6 is more about the looks than anything else, pick the one you like the looks of the best, not much difference underneath, the 4 cylinder and USA spec 6 make about the same power, the six is a little quieter and smoother, but I personally think the smoothness difference has been greatly exaggerated.
The TR3 (never had one myself, but my brother has had one for 30 years, I have driven it many times and gone with him on long trips in it) is a much more involving ride, more wind, more bumps, more noise. For some it is invigorating and if some vintage sports car experience is good, more is better, and you get more with side curtain TR, for some too much of the aforesaid wind bumps and noise.