Opex and other mind-noise comments then, from recollection at the shop:
Expect annual to be in the 10-12k range. Somewhere about 2.5x a Comanche. (we maintained a few PA24s too, so maybe that will help calibrate for your local mx costs
)
IO-520C engine doesn't really have any big gotchas. Expect a top overhaul midlife because continental has their head up their empennage and can't get valve guides right. On flight test, look for at or near 27.5gph on takeoff. This is about 1gph more than the book "m0 procedure" maximum and part of the GAMI religion that I personally follow. 29.5gph if it's 550 equipped.
58s should get 190kt at 25gph or 180kt at 22. Aim for GAMIjectors and she'll do LOP well down to 80dLOP. Sweet spot for this motor is about 8-10K MSL.
These often have cracked spar webs and doublers (far more common than the 55s which don't solve the angel flight mission). Look into the AD history. It's about 6 grand per "corner" now to add the doubler.
Since ergonomics came into the conversation, I find 58s and 36s to have less headroom than 35s and 55s. You should definitely try it on to make sure it works.
Useful loads are ALL OVER THE MAP on these. Start there on your prebuy/due diligence. A light baron is worth waiting for, some are missing 400-600#, either from ridiculous mods, upgrades, bad repairs, or ??? -- early 1970s barons seem to be the lightest and worth a premium to me that the market doesn't value.
I assumed your aversion to the 300hp version is because you don't want a 1984+ 58 (I agree), but I would say the 3-blade-prop IO-550 STC (D'Shannon Raw Power and certain Colemills) are worthwhile. The 4-blade prop colemills are worth avoiding as the props are unique and spendy to OH.
I would jack the thing up on prebuy and verify the gear rigging via the ABS Gear Rigging Guide. They don't go out of rig very easily but a decade or two of neglect can find some real dangerous configurations or need of expensive parts.
If you're giving it an eyeball, make sure the flaps (when up) are flush with the bottom of the fuselage, and the ailerons when neutral are flush with the flap AND wingtip. If not, look for damage history or serious rigging faults.
Manual cowl flaps are a bonus if so-equipped. The little motors are unobtanium and the silly ram-air only gives like 2kts. (scoops look cool though)
Tell your insurer they're way easier to land than a PA24 so you should be able to skip any checkout requirements
Expensive/irritating jobs on these:
Starter adapter replacement/overhaul
Control cable replacement when they get gritty/beyond pressure-lube-able
Anything under the dash honestly. It's 10# of ish in a 5# panel
Front door hinges and door latch rigging
Cabin step attach fittings run loose, the step can corrode internally, and the entire floor area where the step attaches can just get sh*tty. Check for weakness and rust and prior welds on the step.
There is an SB for the RH flap ribs cracking because of walmart nation types jumping on the flap when boarding. about 4 grand to reksin the flap.
Fuel bladders! (4 of em!)
Worth doing:
Replacing the "live fuel line" FF gauge with a Shadin and remote transducers. Fuel lines in the cockpit can chafe and ruin your day
Lord shimmy damper instead of the OG beech
Get a redline sidewinder tug for the thing and just keep it in the nose. Pushing Barons around on the ramp with the little blue cessna bar sucks and is the opposite of fashion.
Plan on a continental "M0" fuel setup twice per TBO cycle. The fuel controller is a rube goldbergian nightmare of failure and nonsense that goes out of rig quickly.
"Otherwise, Barons really are trouble-free!"