In nearly 6 years with the TB20, from new, my only significant bill so far has been a new battery, at about £300, and that one lasted a lot longer than it should have.
There was a large amount of in-warranty work on the mostly American-made avionics, the reason for which was never determined but I have one or two ideas... but all planes have much the same avionics, and if you go to glass then a whole lot could go at the same time. The only way to definitely avoid avionics suprises is to not have any avionics.
If you buy new, and it's a good proven design like a TB20, you should not expect major bills for 10-15 years. After that, airframe parts will start to go, and they are always expensive no matter what it is. £7000 Annuals can be normal for a very old C150...
So, buying something say 25 years old is merely trading initial expenditure for continuous expenditure. To get the best out of the initial depreciation, the best buys are well under 10 years old, and hangared. Damage caused during maintenance (usually the main reason for airframe damage) excepted, mine still looks like new.
I have never flown the Commander but looked at a number of them when looking in 2002, and all advice I got from maintenance shops was "avoid", due to maintenance complications resulting in long time spent on the ground. One I know of has been grounded for 6 months waiting for parts. I fly a lot and would find that absolutely unacceptable.