I always understood the Ingersoll story differently. Colt was bought out by JI Case (the Ag tractor manufacturer) in 1964, and the tractors were sold under the Case name. Jack Ingersoll acquired the Case line in 1983, and began labeling the tractors as Ingersolls. He sold the company in the '90s and it subsequently went bankrupt. I do agree, they were stout tractors back in the day.
I have a preference for Deere garden tractors, and have been hunting a Deere 430 for a couple years. I'd suggest checking out Weekend Freedom Machines, and MyTractorForum. Both are gathering places for garden tractor collectors and have classifieds - Weekend Freedom Machines being Deere only.
👌TWG, I did forget to add the Case branch on the Colt/Ingersoll family tree. I also agree the JD 300/400 series are some of the best GT's ever built. I had a 332 JD with the Yanmar 3 cylinder diesel with ag tires, 3 point hitch, front mounted dethatcher, bagging system, cultivator etc. I sold it all for $4,000 several years ago. It was a real stump puller.
Sorry, double post.