Looks like the engine date code is in the range for 64. 4(64) B (Feb.) 12 (12th day of the month) Th e cyl head engineering #'s the same but he misread it. Should be C4AE 6090 O. I would double check the last "O" to make sure. There's also a date code on the heads between  the middle valve springs. I would think they would be close to the date on the block.
Hard to say what it's worth without at least seeing a picture of it. Sounds like it needs some work and you could open up a can of worms.
I'd start low on an offer, you can always come up.

1st thing is a rear seat, only the bottom seat part. I have a 61 Starliner and mine has a different seat bottom than other cars. Most car rear seat bottoms have a rounded area where it sits over the driveshaft tunnel. Mine is flat all the way across and sits on a raised panels each side of the tunnel. Haven't checked anywhere yet for a seat bottom been busy getting the engine back in the car. Thanks for any help.

Maybe try bending the ends of the lock ring so it fits tighter.

I picked up a complete interior kit from Labarron Bonney a couple years ago for mine. Fit great and looks good. Problem is Labarron Bonney filed for bankruptcy about a year ago. You might try SMS industries for an interior. They are the supplier of materials for most all of car upholstery shops. Trouble is they are notoriously slow. If your not set on original patterns for your interior sometimes you can choose something close to speed things up. That's what I ended up doing because SMS dragged their feet over a year to supply one grain pattern for my seats. I ended up choosing the same color but a different grain for mine. Labarron Bonney finished it in a couple weeks. I can't tell the difference, it looks and fits great.

Welcome to the forum. I picked up a 61 Starliner a few years and here's what I ran into with mine.  On the interior the PO made his own door and quarter trim panels. Best I can say about those is they covered to doors and they looked homemade. The seats he installed 65 (I think) Tbird bucket seats in front and a 64 Galaxie rear seat. Rear seat would have looked great but it was in poor condition. The buckets weren't very good either and they didn't fit the floor right. Had to make a shim to make both outside seat tracks fit right. My solution for the interior was purchase a new interior from Labarron/Bonney. Of course that required finding original seat frames for it. Fortunately a friend had a 61 in his collection i horse traded for some seats. You have to watch out what seats you find because there are different seat bottoms depending on how your floor is configured. Some have a flat front floor, others have front floor wells. I still have the incorrect rear seat bottom in mine. Mine now looks factory stock if you don't look too close. I'm not sure about swapping different year door panels and seats. PO made the rear seat of a 64 work but it was really tight when I disassembled it.
The engine/trans. Mine came with a 406, 3x2's, and a toploader 4sp. but currently gathering parts to make it a 427. Personally I would go with the 428. It's a bolt in. Trans, you could go with a C6 if you don't like to shift gears (clutch linkage is hard to find and $$). An early AOD would work with an adapter and a little fab work. I had a T5 in my 57 Ford and it was a blast to drive. I'm sure it will work in a Sunliner along with a toploader or a T10.
Do your research and ask questions here, I'm sure others can chime in with answers.