I did a lot of research on E90 BMW's as I almost bought an E91 wagon after VW bought back my TDI.
The inline non turbo 6 in the E90 BMW's is actually a rock solid engine that is tried and true and very reliable when properly maintained. This is one of the last great naturally aspirated engines with everything now going to less reliable turbo charged inline 4's. The engine has a great design and makes the right noises. Torque is linear and plentiful. Some of the earlier versions had ticking in the hydraulic valve adjusters, but this was remedied for cars manufactured after November 2008 - so early 2009 models could be afflicted. Something to ask about.
The turbocharged 335i models are less reliable and more maintenance. I would avoid these if you are looking for reliability.
Overall, the key to keeping maintenance costs low is either doing it yourself, or having a good independant shop. A BMW dealer is going to charge very high prices for maintenance. I'm guessing Denver has some good options for BMW maintenance to avoid the dealer route.
You often find BMW's for sale with 90-100k miles. There is a reason for this.... there is a lot of maintenance needed around this point. Service records are helpful here, if you do not have them you need to replace a lot for piece of mind. Flush all fluids (coolant, tranny oil, brake fluid), replace plugs, filters, etc.... I would budget at least $1k for maintenance the first year. With the car only costing $9500 you will still be in good shape if you need to spend another $1-$2k the first year.
If you are used to a Tiguan, at least your expectations are lower than Honda owners, so the BMW should be fine. Have it looked at by an independent mechanic before buying/
My budget was closer to $20k. For me this would get me an E91 with 75k miles or a Certified Pre Owned 2015 GTI with a warranty and much better mileage and more power, so I went with the GTI. I do love those BMW's though. Good luck!