I will be buying a 1988 N/A supra in 2 days, 1 question

[CyFi6]

you can pull the EFI relay which is located in the fuse box under the hood, on the drivers side. That will disable fuel for you. Things you will want to do right off the bat
Oil change-for sure
coolant- for sure
power steering- would be a good idea
tranny fluid- would be a good idea. Use Redline MT-90
Diff fluid- would be a good idea.
Brake fluid- a must if its looking old or dark
check brakes for wear
Check all suspension components and replace as necessary

You might want to ask when and where the head gasket has been changed if they are the orig owners. With 185k miles i am assuming it has blown the head gasket sometime in its life, and if it hasn't, i would be prepared for it to come along soon.

Also: Lift up the wood panel in the trunk and look in the spare tire well, where it sits. This is a spot known to accumulate water from leaky hatch seals and get rusted out.