Good morning Phil,
First of all stand back and look at it. Is the bonnet (hood) straight and in good condition? Read its history from the appearance. Look at the tyres all round espacially for any cracking in the walls. How much wear is present on the pedal grips? Are the linkage balls in the lift arms loose? Is there any excessive wear in any part of the 3-point linkage? Check the brake backplates for any sign of oil or dampness caused by a leaking seal. Are the brakes satisfactory?
How much play is in the steering? Check for wear in the front axle pivot bush and pin and steering ball joints. Check for any gap under the steering arms on the front axle. A gap of over 1/8" would suggest wear in the thrust bearings. Is there any play between the drop arms from the steering box (one against the other)?
Find out what has been done to the engine with receipts if possible. Check for good starting and no or very little blowback from the dipstick hole. There is a bolt at exactly the 12 o'clock position attaching the gearbox to the engine. Check if it is there. Many leave the bolt out because its a little awkward to fit. If the bolt is there then for me its a sign that the repairer has be conscientious enough to fit it.
Drive the tractor in all gears and apply the brakes firmly but briefly and listen for any strange noises. Check smoke emission when doing this.
I don't have the pound/dollar conversion to hand but in the UK I'd expect to pay between £2000 to 2500 for a good 35. If it all checks good and you can buy it for close to $4000 you should be onto a winner.