I LOVE 90s Caddies. I currently have 2 last gen Eldos and have owned 3 N* Caddies before that.
They are great cars and make very comfortable versatile daily drivers. Best FWD cars ever IMO (tied with 07 Monte SS) . ... Not that best Front drive means a whole lot. AWD/4x4 is always the ideal for bad weather but the FWD is nice if you live in a versitile climate that freezes over alot in the winter (midwest) . Trying to get up a steep slippery hill in a RWD car can be a real pain sometimes... especially if its a powerful car.
OP, I can probably help you with any questions you might have. Is the DeVille youre looking at a 4.9 (94,95) or a 4.6N*(96) ?
Its the 4.9. You hit the nail on the head, I have a RWD car now, and going up all these steep Pittsburgh hills is incredibly annoying, and when winter comes, I'll be spinning tires alot. I just love these Caddies though they are so big and comfortable!
Yeah they are extremely comfortable, like driving on a cloud

and the N* variants are suprisingly fast for "grandpa cars"

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The 4.9 block itself is generally regarded as being much more reliable than the N*. Its no secret that pre 2004 Northstars do have a pretty high "headgasket failure" rate. Not actually a failure in the gasket itself but a design flaw where the teeth on the headbolt were made too fine. A proper fix invloves installing oversized headbolts or studding the engine. Once properly fixed a N* is usually good for at least 150k but buying a car with existing HG problems can prove to costly.
The Hydramatic ******s are extremely durable unless someone has really abused the the car.
Lol @ 9 mpg. I dont exactly know about the 4.9 but my N*s average 20 and I drive with a lead foot. I would think with the extra displacement the 4.9 would use alittle more gas but not that much.
3k is a fair price but I would try to knock off at least $100 just for the sake of the haggle. The car looks very clean , as do most low mileage Cadillacs

. Thats one very nice thing about buying a used luxury car regarded as a "grandpa car" ... Grandpas take good care of their cars