Welcome to the forum!
As project_x said, there may be some age-related issues with the machine. It's 25 to 30 years old, after all. If you were looking at a car from the early '80s, how would you expect it to look? If it's pristine and seemingly untouched, it's obviously been redone. Some of the machines are treated in Vietnam (or other places) to a "reconditioning" process that has a bad reputation among the members on this board.
Some have had horrific experiences with tractors done that way. I think I have one, but it's actually the best machine I own. (Though that isn't saying much.)
Look for obvious leaks, drips, cracks, or breaks in things. Check the play in the steering wheel: if you lightly press one way on it, how far does it go before the wheels move or the steering wheel gets tight? Does the steering arm move much before the tires do?
Examine the knuckle castings and axle components, and really the entire frame/casting sections of the tractor for fractures or bends. The front end of these tractors really take a beating, especially with a loader.
In that area, I would look even more closely at the air filter than I normally would. (I used to live east of there about 100 miles, and my brother is in the area) Pull the cover off and look down into the manifold to see if it is passing contaminants through or around the filter. The loess soil there, combined with the prevalence of agriculture in the area, puts a lot of dirt into the air. Aftermarket filters for some Yanmar machines are notorious for not sealing properly and passing dirt. I see the 1602 has a canister style filter, which is good, but still check it.
Check the fluid in the transmission after running it around and cycling the loader and lift. It should be clear to brownish, but look clean and transparent. (Rum is brown but clean looking and transparent, while pond water is brown but dirty looking) If it looks whitish or cream colored, there is water in the system. It's not an emergency or a deal breaker, but needs to be remedied. Find out how long it has been like that.
Try to start it cold, without being "warmed up for you" or pulled out already. A puff of smoke when starting is normal. I don't have a 1602, but have some moderately comparable ones. My 3 cylinder engines fire up easily cold after running them with the compression release activated and the fuel on until oil pressure is accumulated. My low-ish compression 2 cylinder tractor takes a couple spins around with full compression to build heat, while my tight 2 cylinder tractor bangs to life instantly, even with a pitifully weak battery that can't spin the engine through against the compression. I've never used, or needed, a pre-heater or starter fluid on them. Ask if he needs either of those in winter. (You want him to say he never uses starter fluid.)
The weather in Yakima/Selah/Naches will be more than warm enough that it should come to life willingly, even after sitting for a few months. If it doesn't, something is amiss (I won't say wrong.)
Operationally, check that it goes into and out of four wheel drive. It might be slightly tight going in or out of the position, but it shouldn't require a boot or a smack to go in or out. The same goes for the high and low range, each gear in the transmission, and the PTO drive.
Ask him if the tires are filled, if so, with what, what type of oil and filters he uses, where he sources them, and so forth. You'll be able to figure that out yourself, but it's a good way to get a feel for how well it has been maintained. Ask for any manuals or brochures he has, where he originally bought it, and so forth. Have him show you how to set up and operate the mower, in particular, and at what PTO speed. See if he has belts for it, too.
When you are smitten by how awesome a machine you're looking at, make sure to take pictures of the area and the machine and you picking it up, and post them here! Give it a good bath just in case it's been worked pulling a pesticide sprayer through the orchard. And then go play with it! I really like the size and look of the 1602 and 1610 tractors. I haven't yet found one for myself.
You'll get off the hook for me asking you to post the model number and specifications for your pump, since project_x already did so for his.
Good luck!
