Hi James, Throttle control should be lever on dash on right side of steering column. If there is similar lever on left side of steering column that is T/A control, forward is direct drive, back is low. Re gear shift levers: could be two different styles, old style: lever has 4 positions, gears 1 to 4, 1 being at top rear and 4 being at bottom, the other lever selects Rev, Neutral, Low all the way forward and Hi half way down and towards drivers seat.
New style has lever for 1-2 and 3-4 in H pattern and other lever is Rev, Neutral, Hi. Low.
Re weakness, power brakes and parking brake.
Check PB individually for being "spungy". Could be caused by leaking PB cylinder O-rings.
Check parking brake for holding ability. Problem is Operator could have driven tractor with parking brake engaged and chewed up parking brake band in the diff and all the bits will clog up the hydraulic filter.
Pull the dip stick and check the crankcase oil for smelling like Diesel fuel. Could have the front drive shaft on injection pump leaking Diesel fuel into crankcase.
Otherwise a very good 40HP tractor and the German made D-179 engine will run a long time if properly maintained.