It's not so much about upgrades. It's about the ROI, which includes the value of time.
…Where is the break-even point?
That’s a personal choice because it’s a measure of profitability, which is irrelevant. You’re assuming the binary choice of restoring/upgrading vs buying new.
There’s a third option, buying something that’s already had all that work done. Discount wise, that’s the sweet spot. Premier Aircraft Sales brokers stuff like this, and unless you’re already on the wait list or buy a spot on a wait list, there ain’t showrooms of new planes sitting around, unspoken for.
Ex: a clean, 2002 A36 with a mid-time motor and well equipped is going to run $400-$500k. A mid-time G36 is $700-800k. A new G36 isn’t being made.
A clean, mid-time, well equipped Ovation 2 will only set you back $250-300K. No Mooneys have been made in years.
SR22: mid-tine G5 NA is $500-650k. new G6/7 is $850k-$1.4M.
But none of those guarantee dispatch and 100% uptime.