Bubbles: Look at it from a logical point of view: Why are bubbles in the radiator? How do bubbles get in the radiator? There is no mechanism in the cooling system that would generate bubbles, swirling fluid that comes and goes as the stat opens and closes, but no bubbles, especially continuous bubbles that come when the engine is running and cease when the engine is off.
Enter the combustion process. Now we have compression in alternate cylinders that exceeds a couple hundred psi at least....don't know but cranking compression gets you up to 150 and even at that that's plenty to push combustion gasses by a leaking gasket and into the coolant.
I'm going to stick with the head gasket leaking, not much, but still leaking.
On the oil, again, I have never seen yellow oil in a normally operating engine. However, I do not live in the north and do not have some of the high temperature swings you get up there and thus may not have been exposed to some of the limits of condensation.
I still think that the engine is defective and if the price is right and you feel benevolent towards the seller then bite the bullet, know that you have some work to do, how much is the question, and get on with the sale.
I bought a 4 cyl 4000 that wasn't running. Seller said make an offer. I offered $1000 which turned out to be too much but I made the decision at the time to go for it. It had a FEL that would bring half that so it wasn't a big risk.
It had no oil pressure which he identified at the time of sale from the last time he ran it.
In short, the engine was shot, mains toast and no oil press due to sloppy mains. I had hoped it would be something like sludge.
I overhauled it for about $1000, sold the FEL for $500 and wound up with a nice $1500 tractor.