As always, buy the best available bike and not a cobble up of many bikes. I prefer the oil in frame bikes as they are more comfortable. 1971 has a high frame which is why i like 1971 models. I cannot ride a left foot gearchange T140 as my left foot does not bend. I suppose the later T140s are better due to having brakes that will stop the bike quicker than my drum brakes. Never found any problem myself with the conical hub on mine though. A T120 is high compression and the T140 is a bit lower. I think what i have with a T120 and a 750 big bore kit is the best set up for fun.
If buying one now, I would buy another 1971 bike.
Possibly a later model as suggested might have some better equipment.
It took quite a few years to get mine how i wanted it.
More important is that you do find the very best one in your price range from an owner who knows the bike and is a competent builder.
I use an external filter which can be changed in a few minutes so would not fit one inside the frame.
The Tiger single carb bikes will be almost as fast as a twin carb and less maintenance.
Go out and have a look at a few. I took 3 months to find mine and have owned it since 1998. I did own an identical one before this which was written off.
Watch for any oil leaks near the swing arm mount. Some early bikes cracked at that point. Later ones had reinforcement added.