I agree with the comments above. It depends what you want your tractor for. Grey fergies (the T20) have a number of limitations for practical use. The pto and hydraulics are linked to the main clutch and hence stop working when you depress the clutch..this was common on early tractors of all makes---later 'live drive' was introduced by all manufacturers. You need to move to tractors after about 1965 to be safe on this...in the fergy range the T20 never got live drive, the 35 was available in both forms --you had to have a delux model to get the 'live drive' (it came with a better seat and a tachograph too !). It wasn't until the 135 came out that you had live drive as standard.
Another thing with the T20 was the hydraulics are only draught control not position control...which means implements are either fully up or fully down, this can be more of a pain than you might realise. With the 35 came position control as standard.
In the fergy range id try and stretch to a 135 (if you need the practicality) if money isn't an issue .. prices are high, but they hold there value and go up.--spares for all the fergy ranges are cheap and readily available...a major plus point.
If cheap is the key ..I have limited knowledge of other makes, but the international 434 is an extremely good tractor, strong as an ox and not so collectable as fergies. Ticks the live drive and position control boxes too.
Depends what you want..what you plan to use it for and what your budget is.
Finding decent tractors is always a problem, especially if you are unsure what you are looking at. Most tractor dealers offer no warranty on tractors of such an age..but have added on there percent. Ebay ..you can often spot the genuine ones..been in regular use looked after by an old guy who has passed away and family now selling. On farm auctions ..again its risk but a genuine retirement sale can yield good stuff. Collective sales..usually where everyone offloads there crap..tread very carefully.