We had J's and XL's so don't have any first-hand experience with the K's but have heard the fiberglass doors can be a real P-I-T-A due to warping and sagging.

You should be able to make pretty close to the same weight bales out of the K as might come out of any newer baler as long as the air bags, belts, etc. are still good.  So if you're inclined, you can really load up a 3/4 ton pickup if that's what you want.  Neighbors had to add air bag assist to their 3/4 ton '99 Dodge with hydrabed (hauling 2 "XL" bales) when they went to a slightly taller tire than the originals.  They seem to get along decent with that setup.