I understand where you're coming from. A 128i maybe not be considered a "serious enthusiast" model by some, but who cares? That N52 is a fine engine and it offers plenty of power for a car the size of a 1 in 95% of driving conditions. It's not a high-speed track beast and it is limited in terms of engine tuning options, so you have to be willing to appreciate it for what it is and not worry about what it isn't.
That said, I don't think an automatic transmission is a great match for the 128i (are you thinking auto or manual?), and the extra weight of the convertible doesn't help either. Before I bought my 335i Coupe, I test drove a 135i Coupe with the regular automatic transmission (dealer had no manuals, of course) and it was a winner. I'd still prefer a manual in any situation, but the N55 and an automatic are a fine match IMO. I'm sure DCT is that much better, but I've never had the opportunity to drive or ride in one.
I know a few friends who are perplexed by my decision to turn in a 335i Coupe and replace it with a 128i Coupe because they automatically think that more power and a higher number on the badge are always better. I don't care about status or being the fastest guy on the block -- I just want a traditional, nimble BMW with a naturally aspirated engine because that is my strong preference. There are certainly some aspects of my current car that I'll miss in the 128i (even ignoring the additional power), but I know myself well and I know this is the right move for me.
And that brings me to my point: IMO, the most important thing is to know your own value system, stick with it and don't worry what anyone else says or thinks...including me!
Seriously, it seems like you know what you really want, but you're also trying trying to take into consideration your experiences with other CCA members. That will probably result in a compromised decision, and for what?
If you really must have Blue Water -- and again, I understand that sentiment because I *love* the color of my current car and I would buy another Blue Water car in a heartbeat -- and you want to stick with a convertible, the decision about engine and options may almost make themselves. 1 Series are not common in the U.S. and Blue Water is definitely a rare color, so you may need to keep an open mind or consider other colors. If you must have Blue Water, I'd start searching now because they're only going to decrease in number from here forward.