Sorry to be so late to the party here... since this site is so new, I frequently forget to come and see what's happening
Anyway, I think we need to remember a few things, certainly as it relates to the early Alpinas like the B7 Turbo. 1) the 'M' cars from BMW didn't really exist in that body style at that time (other than the M535)... so there wasn't really a choice if you wanted the highest (at the time) performance from an e28. 2) Alpina was (and still does) also offering other things beyond engine performance to differentiate their cars from a standard BMW offering, e.g., suspension, brakes, exterior and interior aesthetics, etc.
It is certainly true that one can take just about any model of BMW and with good and proper after-market parts make it as fast, or faster, handle as good or better, brake better, and maybe even look better... and that's fine and I understand why people do it (guilty as charged
) It's also likely that you'd spent less $ than the cost of the "M" or Alpina, and I'm sure it will be an enjoyable car, just know when it comes time to sell your modified car, you clearly will not recover much of your investment, and your car will probably be worth less than the stock model it started out as. BTW, I think personalizing one's car is great... I've done it to all my car, but there is a price to pay along the way and in the end
So, in my case, I purchased my Alpina for many reasons, and in the following order: its rarity, its look, its performance. Likewise with my e28 M5...same reasons. I enjoy both and appreciate their similarities (many) and differences (few). But, I'd prefer to have the real thing over a clone or modded car... even if the that car is faster or performs better than an Alpina or M car. This, for me, is even true as it relates to a car from a different era, i.e., I prefer my e28s to a modern BMW like a 335i... but, probably not a new M3 or M5
Bottom line I'd say is: if you can't find what you'd like because its so rare or you don't want to spend the big $ it might take to get one, then modding a base car is a great way to go.
-RoyW