Your membership in the Black Sheep Squadron has been duly noted and would have been approved by the higher ups if we actually had any. No big shots in the black hub club though. You will be required to get drunk and ride the donkey at the next meeting.
I've had to address these issues with my 400's..
Engine: two of my K532's powered 400's broke a rod before I acquired them, which is actually why I bought them. The price was right even after replacing the Kohlers. I only paid what the deck was worth for one of them. The other one was part of a package deal (deck, model 50 blower, 4 way 54" blade). It has a really low hours K582 now that was about half the cost of a repower. The other one has a 23 HP Vanguard. I like the 24 HP Honda engines too. I'll take either. Can't get the Predator clones here though because of an emissions issue.
Driveshaft: if it still has the original one, expect to replace it. Look for play in the knuckles and reddish dust on them. They aren't serviceable. I made a new greaseable shaft last year for about $150. I'm in Ontario and shipping was a little spendy for the knuckles, plus the currency exchange is about 30%. You can no doubt do much better on the cost. I'd estimate a hundred bucks in the US but don't quote me on that.
Oil cooler: I only have one (out of 4) that wasn't damaged by the driveshaft knuckle. If the bolts for the cooler loosen up it will drop down and the knuckle will start grinding on it. That can wreck the tubing pretty quick. I bought a brand new one when I did a resto on that 400 (ouch $). My 400 with the Vanguard has plenty of clearance though.
Edit: the Vanguard actually sits higher than it should. The rear mounting points for the cradle are 1" too tall so I put spacers under the front mounts instead of shortening the rear mounts. So if it was sitting where it should be there wouldn't be much clearance for the cooler.
This is what happens to a cooler because the bolts were loose. I had one that was even worse than this but no holes in the line yet. I replaced it with a better one but I didn't get rid of it. It's on the Just In Case shelf where I keep stuff I might need but likely won't.
Steering linkage (bushings, bearings). Another PITA type of task but not expensive. It's actually pretty easy if you have the tractor apart. Some of the tie rod ends will likely need attention.
Steering valve: about 10 bucks in parts. Two seals and a backup washer. Easy to do except for the inner seal. Sometimes there's rust on the rearward end of the valve and there could be some pitting. Two of mine had that but were still good enough to continue using. Edit: forgot about the spring breaking sometimes. That info was courtesy of Mr Wizard (aka Harold Goff). He has detailed knowledge and experience with 400's. If the manual says one thing and Harold says something different I automatically assume the manual is wrong.
Steering cylinder: I have never had any issues with them but they're the same as any other cylinder as far as wear and tear (hoses and seals) goes. I had one break the tie rod end where it bolts to the tractor.
Hydro control linkage: mostly just a case of replacing bushings, but the pins and the shock absorber might also need attention. I also replaced the lever itself on one 400. The original one was worn where they pivot at the bottom. Made the new one using rod of the applicable size. Definitely helped. I consider it a must-do when rebuilding the linkage. Otherwise you might not see any improvement in the lever control.
Front PTO bearing: simple and inexpensive (relatively speaking). Same bearing as the deck spindles.
PTO idler pulley/linkage: the pulley can require replacement (same as any other idler) and the linkage gets loosey goosey after a while. PITA to fix but not spendy at all.
PTO cable: they can break. A new cable was about 60 bucks + tax last time I checked, so about $70 total. They have no doubt increased in price since then. A thick-ish clothesline cable works great as an alternative. Just gotta make a loop on each end and presto. And cable clamps are only about 2 bucks per so that was an easy call, for me anyway.
Hydro pump. I replaced the pump on one 400. It was noisy (howls somewhat) but still in working condition. I have it stashed in my JD parts cabinet ..just in case. The pump to transaxle gaskets are NLA so I made one. They're like valve cover gaskets. No pressure to worry about. Just a case of sealing up the mating surfaces.
Hope that helps.