I grew up on a Ford 850 and own 2 versions of the later 4000's today.

I'm going to answer (C) first! Comparing the 800/801/early 4000 series tractors to the 65 and later 4000 series is like comparing apples to grapefruits, their nothing alike, the later 4000's are bigger, heavier (over 1700 lbs), and stronger built than the early 4000's.

A more equal comparison the the early 4000 would be the 65 and later 3000 model, same basic chassis but with the 3 cylinder engine.

Now back to (A) I consider the 3 and 4 cylinder gas engines to be equal in durability but the 3 cylinder models governor system is a bit finicky and the ones I've driven where not as responsive is the 4 cylinder models. I consider the 3 cylinder diesel to be superior to either of gas engines and early 4 cylinder diesel.

(B) 801 and early 4000's where available with power steering which is the same system as used on the 3000 models but wasn't used on the later 4000's.

Here's some comparisons.

63-64 4000 - 172 ci engine rated at 46 hp, swept back front axle, 13.6-28 rear tires, single reduction rear axle with drum brakes, live pto with 2 stage clutch, weighs around 3200 lbs

65-up 3000 - 158 ci gas or 175 ci diesel rated at 40 hp, swept back front axle, 13.6-28 rear tires, single reduction rear axle with drum brakes, live pto with 2 stage clutch weighs around 3700 lbs

65-mid 68 4000 - 192 ci gas or 201 ci diesel rated at 45 hp, straight boxed front axle, 16.9-30 rear tires, planetary axles with wet disc brakes, hydraulically controlled independent pto weighs around 4900 lbs

68-up 4000 - 201 ci gas or diesel rated at 52 hp, other specs are the same.

The 3000's extra 500 lbs is the additional gears in the trans and the much heavier built 3 cylinder engine placing most of the additional weight on the front axles, this makes for a better balanced tractor than prier models but increases low speed steering effort, for is reason I consider power steering a must for the 3000 and later models.

65-69 4000's used a manual steering box with a externally valved steering cylinder, this worked well but was slow and it is easy to out run the hydraulics even at operating speeds.

70-up 4000's had the much improved integral power steering.

All this said there is the 1 oddball, the 65-up 4000SU for special utility!
It has the same engine, trans, planetary axles with wet disc brakes and ind. pto of the 4000, but has the swept back front axle and 28" rear tires used on the 3000 models.

Here's a few pics of my tractors.
62 801 diesel with factory power steering



72 4000SU, same basic size as a 3000



69 4000 all purpose with later model flat top fenders



Disclamer: The tire sizes I listed are what is normally found on those models in my area although other sizes where available and could be more popular in other areas.

Example: 13.6-38 rear tires where more popular on the 65-up 4000's in some areas but I've only seen a few around my area.

Although I like the -000 series tractors I have, for a first time upgrade if money allows I would recommend looking at the newer 3600/4600 models.

Dry element air cleaner over oil bath

Relocated and more user friendly parking brake control

Engine driven tach over generator driven

4600 has same integral power steering as found only on late model 4000's

BINGO!

I tried for a several years to find a good 4000 SU (preferably) diesel with either S-O-S (I like them) or 4X2 transmission before giving up and buying a new MF 533 (the 2007, 201 CI Perkins modernized version of the 135). I wanted the LP SU version of the 4000 for mowing duties on steep ground but simply could not find a decent one.

Subsequently, I bought a second, new 60 PTO HP MF because I wanted to keep my local CNH dealer honest.

Ultimately, both MFs turned out to be mistakes and I sold both at losses, replacing both with Kubotas. Since then, I have bought three more Kubotas and there is no going back.

Dean