TBE never had an HBV failure, although I always kept a spare in the boot, along with a spare crank sensor, although that never failed either.
The Silver Bullet had at least 1 HBV failure (might have been 2). But then it also had 2 crank sensor failures. Oh, and 2 head gasket failures
None of mine had oil cooler failures, though I bought The Heap (N reg 2.5 CD) with a failed cooler, hence dirt cheap. I'm fairly sure that nice DTM chap told me to bring a new oil cooler when we changed the heads over at the 2nd Telford meet, though the one one was fine, si the Silver Bullet did get a new oil cooler.
As a general rule of thumb, I changed my coolant in the Omegas about every 2 to 3 years, using the dealer supplied GM stuff based on low price (something like £20 for 20l) and convenience (delivered free, usually same day). 15 mins of work and about £6, gave me something to do when the oil was draining