The amount of trouble you'll have with rust depends on your local climate.
Other than that, they're not designed or built very well, which shows up as bits breaking and bad electrics.
As with a 147 you must replace the timing belts and tensioners on schedule (every 60,000km), but you probably won't have to top up the oil as often!
Carby 33s are prone to not running 100%. Problems like refusing to idle on a slope, stalling, idling rough, jerky low-speed running, missing at idle and light loads etc might mean a trip to a carburettor specialist and the extra bonus of recovering some lost power.
33s are pretty basic though and mine have been quite reliable considering how old and rubbish they are.
I'm hanging on to a 1.5 twin carb 33 at the moment that I just can't bring myself to sell. It turns you into a race driver, because
* the engine sounds great
* there's not much power so you need to race but won't find yourself speeding
* you must double-clutch because the gearbox is bad
* the engine runs best at high loads or rpm thanks to carburettor problems
* the lack of refinement and comfort don't suit any other style of driving
I have a review from back in the day which says "cracking crisp engine note will make you smile". That's not all it will make you do - you'll be creating situations where you 'need' to put your foot down, racing everywhere you go, you'll be forever throwing the car lifelines to keep it on the road, and you'll start hearing engine noises in your head. BrrrraaaaAAAAAA-bzzzzzzztinktatink...
Nobody will understand when you tell them they'll have to prize the steering wheel out of your cold, dead fingers..