The e46 M3 can be quite reliable, if you're willing to spend a weekend (or pay someone) to lock it down. Takes ~$500 in vanos lockdown parts and $120 in subframe foam (assuming you buy one where the subframe hasn't failed-- the subframe gets quite expensive if you allow it to fail). SMG isn't the reliability disaster it used to be-- the pump can now be rebuilt on the cheap (instead of having to be replaced), and if you relocate the pump, so it doesn't overheat, it won't fail in the first place (I'd still get 6mt).
That said, all e46s, M or otherwise, are at least 12 years old at this point. If you get a car that has had deferred maintenance, you're going to have a lot of catching up to do. Here is a list of what the e46 m3 needs when:
http://www.m3forum.net/m3forum/showthread.php?t=190111
^this all goes for the ZHP, as well. Really, this goes for any car that you want to drive like new (bushings/bearings/ball joints/shocks) and be reliable (cooling system, fluids, etc). The bill will just all come due at once, if you don't have proof that the PO kept on top of the above. Skipping maintenance/lockdowns is what makes the car super expensive to own (and drive like crap, and be unreliable).
I've been DDing an e46 M3 for the last 17 years. My weekend/track (coupe) M3 is at 150,000 miles (including 10,000+ track miles, and making 100hp more than stock, still NA) and my DD (wagon) m3 is at 200,000 miles (a little bit of tracking). Both have been super reliable-- in that time, I've broken down one time (fuel pump). Original engine/trans/diffs/etc in both cars, and both drive like new (or better, in the case of the highly modified coupe). Properly maintained, they're freaking great cars.
That said, you should also test drive one and make sure it's the right car for you. The e46 M3 is, IMO, the more hardcore M3-- it rides the harshest, it makes the most funny noises (that don't indicate problems), it can't hold a steady idle, it makes the most hp/L and torque/L NA, it has the highest piston speeds, it's undersquare, etc. If you're used to more modern M cars, it may come as a bit of shock. But, the flip side is that all of that makes it more rewarding to drive hard than anything BMW has made since.
I own an M54 powered car, also. I wouldn't get the ZHP over the M3, unless you find the M3 to be too aggressive for DDing. I don't think there's any significant running cost difference between the two.
But, it doesn't really feel like it makes sense to have the more comfortable f8x as your weekend car and the better driving experience but less comfortable e46 as your DD :P
... I say hypocritically, as I've tried to go to a newer, more comfortable car multiple times, gotten frustrated by them, and gone back to e46s.
