Info on a slipping trans

Hi everyone.

I have a compound question about a car I'm going to look at. 68 Valiant 225 automatic. I don't know what trans came with the 225.

Seller says the trans slips when it's cold, but then doesn't slip when it warms up. If he drives every day it won't slip, but if it sets more than a couple days it will slip when cold.

It's possible it's just low on fluid. If filled to the proper level, will all be good or is the trans already toast? Could a band adjustment be done to fix this or again is the trans toast?

I have two parts cars both with automatics, that drove well when parked but that was long ago(one a tree fell on the other the primary driver died). What are the chances I can swap in one of these and have a good trans with little or no maintenance? If I change fluid and filter is that a no-no in something that's been setting?

If I need to rebuild, are there parts that make the job difficult or more expensive, or is it a straight forward rebuild. I won't be doing the job, but would like to know what to expect.

I have a feeling I already know most of the answers here, but I'd like to hear some others experiences. I'm familiar with the Turbo Hydramatic. Good trans but once it slips it's done.

Thanks everyone.