I will respond to #3.
Assuming you have manual transmission. IF auto trans ignore this and resolder the relay first after checking the fuse.

Just bought and the work you did probably means the vehicle sat for a while, just a guess. I'd check in this order.

Fuse, #11, roll it in its socket to clean the ends. Typical of this series.
Then, with engine off, ignition on, shift into 4th, push button, no light? Pull lever further to rear and push further to the right. Push button. Dash light comes on then? Replace 4th gear switch on right side of trans.

Once you have the dash light, then drive and see if you have OD. Dash light working and still no OD? After making sure the trans has a full belly of fluid, contact me for trouble shooting sequence which is too long to post here.

At that age, the OD may not work which maybe was part of the reason it was sold. The actuating piston seals of the OD may need replacement in addition to finding the reason the electrical controls for the OD do not work. Just warning you in advance. A PITA to change but not that hard or expensive.

Duane