I think the 330 you're looking at will have the N52 engine. The ticking problem came from lifters/head design. Replacing the lifters with a revised design that has larger oil passages works 90% of the time. Usually the ticking only came when the engine would idle for long periods, then a "spirited" drive would give the heads a chance to drain. It was a black eye for the otherwise excellent N52 engine. For some people switching to a thinner oil fixed the problem. Others went to war with the dealer and came away with new head and valve assemblies. Early on BMW was paying dealers to assemble the entire top end. Now BMW has a preassembled kit that's fast to swap. Replacing the lifters on your own **IF** the noise was present **AND** bothering you would cost around $200 for valve gaskets and lifters. About a 3 hour job going slow.
Cooling system is solid as long as you service it early (the 100k is a joke, and I know first hand the OP has participated in lengthy debates here on BITOG). The plastic components that some of these people are ranting about are plastic connectors (kinda like the ends of an air hose). Heat and age can get to them, but usually not before the hose itself would be due for replacement. The electric water pump was a design to reduce parasitic drag on the engine (and so was the electric power steering which also eliminated the weight and maintenance of power steering hydraulics). The cool thing about the electric water pump is it has a "service" mode which allows you "burp" the cooling system as you refill it. It's very well thought out and engineered. Sadly most old school mechanics are too hamfisted, closeminded, etc, etc to appreciate the engineering.
The water pump itself costs around $450. Being electric the motor portion should last a very long time, but neglected fluid and particulate mater can eat away at the pump assembly. Replacement is about the same as a regular pump, something the OP could do.
I'm not a fan of the BMW coolant itself. I don't understand how it could last 100k miles. It seems to last just long enough to get past the warranty period. Look here for what before/after pictures of the fluid looked like out of my N52 after 3 years and 24k miles.
http://www.zpost.com/forums/showthread.php?t=488476
Make sure that your 2006 is CPO. At that mileage you'll be outside any free/included maintenance, which I doubt you'll care about being a DIY'er, but being CPO will extend warranty coverage on certain items to 2012 or 100k miles.
I used to dislike European brands but now I've been converted. You'll hate the service at the dealer, but I don't think you'll be there given the mileage you're looking at. For maintenance parts prices are about the same price. OEM Bosch plugs were $6 each, air filter was a reasonable $20. The OEM oil filters cartridges are a little more expensive ($10), but they come with a crush washer and new orings for the filter housing, putting a new high quality oil filter cartridge around $8.50. The repair parts, cooling hoses for example, can have about a 20% premium over a cooling hose for a Saturn, but that's apples/oranges IMO. Since the battery lives in the trunk the OEM batteries can be around $200. Generic ones fit, but OEM ones have vents and unique sizes.
The black boxes inside the car can get scary. The TPMS brain died on my car, under warranty, but it would have cost about $600 for the parts and $400 for labor. The mileage you're looking at if the elctronic component was going to die it would have by now.
I'm sure the first thing you'll do is replace every fluid and maintenance part (like I did). After a weekend of poking and wrenching not only do I appreciate the engineering, I have much more faith in the reliability.
As for the 330i there are some great color combos out there. I'm a fan of the Space Grey oustside and Imola Red inside. Very classy. For me the must haves for a E90 would be Premium, Sport, Ext Leather and Winter. And for all BMW naysayers and non-converts: They drive amazing.
