Get a car without rust. Like a car from the south or California.
My 2nd car was a '65 Fastback with a lot of hidden rust that cost a lot to repair. I found out later it was a Michigan car.
My first car was a '66 Mustang, 200 CID 6, 3 speed transmission. Early 70's in Louisiana.
I agree its hard to get in trouble with one of these. I remember trying to find its top speed and could not get to 100. I agree that engine will run for ever, its intake and exhaust are so restrictive its low power is not at all hard on the engine. I had teenage boy dreams of buying all the Clifford Research go-fast parts for the inline 6-
The 4-lug tiny drum brakes are crap. Allowing distance for brake fade is a thing with these cars, I can imagine what would happen with the braking if modern sticky tires were added instead of the slidey bias plys we had back in the day, stop faster but brake fade would be worse.
Forget about cornering, under steer allowance is also a thing.
I changed the transmission synchro's twice in that glass 3 speed and the clutch several times. (I was still learning to drive and I thought I was to be the next Don Gartlis, not that launches were possible since severe wheel hop is also a thing with these cars, actually for many cars of that era, all the "hot rod" guys had bolt on traction bars because of this)
At some point in the mid-70's I procured a wrecked V-8 Mustang and switched over the brakes, put in sintered metallic brake shoes, changed to the v-8 rear end, got a junk yard Toploader 4 speed, rebuilt it, and also I built up a 289 to probably 280 hp or so ( not bad for back then).
I added GT350 front springs, sway bars, good dampeners, traction bars to prevent wheel hop. I ended up with a really loud car, that stopped pretty good thanks to the metallic break shoes, handled somewhat better than the '66 with 6 cylinder, but probably would not have been able to beat the 2009 WRX I had later, in top speed or acceleration and definitely not in handling. I eventually switched all the parts over to the '65 Fastback because an idiot rear ended my '66 at a stop light and caved in both fenders, this absorbed some of the impact, no gas tank leak though a direct hit. By the way, the seats of that era had no head restraint.
Lucky I was young and tough or the whiplash would have been severe - My neck was sore for a only week. 62 year old present day me would probably require surgery and a remaining life time of misery for the same hit in that car today.