with the OP's list of mods, I'd argue that his 325i is probably pretty fun to drive. Maybe not as fun as a BRZ, but I don't think he'd be happy with a stock BRZ and would want to spend more money on mods
Anyway, as far as insurance costs, I was going by actual experience.. insurnance on a 14 year old integra GSR was more than insurance on a 2 year old E90. Both had 100/300 coverage limits. the BRZ is a similar type of car as the integra (as far as demographics are concerned), so I'd imagine a new BRZ would have a high rate of insurance. Though rates do vary by location, and I imagine that living in L.A. had a lot to do with it.
As far as maintainence on the BMW, I have 108k miles and haven't needed anything other than oil changes and one set of brakes. at 100k, I had the trans fluid and pan changed ($240 in parts), and changed the spark plugs. He's got a ways before he needs any of that though.
And I doubt the BRZ comes with free maintenaince, so he'll need to pay for some of that anyway.
I'm sure he knows what the best financial option is, but it really comes down to how much he wants a BRZ. When you really want something, you tend to find a way to justify the purchase, even when deep down, you know it's not the best thing to do.