I think if you are prepared to learn, have time and enjoy learning the technical side of things then getting a "manual" DSP such as the BitOne, Helix P-DSP (£300), Mosconi 6to8 is a no brainer. If you just want to get something to plug it and never tweak it then an ms-8 or an alpine H-800. Having said that you still have to do loads of calibrations on the ms-8 to get what you want. So it isnt as they market it trust me! I'm not sure how the Alpine is.
There are loads of DSPs out there. I narrowed my choice down to the Helix and Mosconi as they were the best i could get in my budget. Only reason the Helix did it for me is that the software is so much more user friendly, for me anyway, and it was a bit cheaper than the 6to8. Oh and there shouldn't really be any need to take it to a shop if you are prepared to learn and have the right tools. Obviously someone who has a lot of experience will be able to do it quicker but you should still be able to get it how you want it to sound.
About speakers, have a look at the 3 way gladen audio set. Comes with a tweeter, 4" mid and 8" underseat. All drop in. I heard this and it gets quite low. You could probably just about fool people that you have a sub in the back. But it will be clear to people that know their sounds that there isn't one. Pm "Mr Singh" (he did my recent IB install). Think he can sort you out with a harness too.
Are you definately sure you don't want a sub? It totally transforms the sound in a way which no 8" can do.
2008 E92 335D SE
2010 E90 320D SE
1994 E36 318I SE
1996 E36 316I SE