Figured I'd chime in here, seeing as I own both, and that's my E46 for sale.
I purchased that a few years ago for about 4k, and have spent over 3k on it.
From what I've seen, the M43 does have a few less hassles than the 6 cylinders, but if the rest of the car needs the same maintenance (shocks, bushings).
The fuel economy is definitely more, 2-3L depending on how you drive it, but it does have almost twice the power.
I also purchased an E30 for 5k a few years ago, spent close the 3k on it in maintenance parts on it, and would say it's gone up in value (mmm E30 tax)
Both have been reliable to drive.
I don't enjoy driving the E30 in the wet, so I'd take the E46 out instead for the points mentioned previous by Olaf.
But when the weather is good, it's E30 all the way. (My wife wasn't too impressed the other night when we went out in the E30 because it currently has no heater haha)
I haven't encountered anything yet that is significantly harder for the home mechanic to do on the E46 vs the E30. But the E46 has been a bit more annoying (Had a leaking oil filter housing unit, no it wasn't the gasket...), but would say the E46 is a better all-round car, but E30 is what I love.
But back on topic
You've take the advice, and looking to check out out physically for rust, as that will be the biggest issue with E30's. Everything else is pretty easy to spin a spanner on.
But depending on what your mate wants to do with it, could empty his wallet quickly (but that's more the point with project cars)