The above mentioned store is fine if you are prepared to pay for it. The installation of an amp and door speakers without or without the underseat woofers are relatively simple. Any decent car audio shop should be able to do it. I installed mine myself, and I am a amateur who is far from experienced, though I did stuff up simply because I did not reconnect a plug back properly. However, no harm done, and it is all sorted.
If the 1 series alpine upgrade is the same as this :
http://www.e90post.com/forums/showthread.php?t=182752
then it looks like it is the equivalent of the lowest range of alpine product. I am sure you can get better after market product for less. However, I do understand the peace of mind you get from a BMW installed product, which is why they charge more for everything, tinting, paint protection etc etc.