The headunits are all the same, there isn't a DSP / None dsp headunit.
I come back the original point, you are keeping the original speakers? so you I assume you don't really need stupid amounts of RMS per channel. Why don't you get a bm54 modd'd by the nice chap in London who does it, he can add you an amp to give 40 watts rms on the channels, this would give you enough for the standard speakers, he can also add pre-outs to the standard bmw headunit (there are guides to do both these mods yourself if you feel you wish to) then you have a headunit with 40w rms on the four channels, you have a set of preouts for the subs. I seem to believe he charges less than £100 for the mods so would leave you sorted for a cheap sub and amp setup.
I wouldn't stick the amp back in the boot pit, I would stick it to the rear firewall - this would cut down cable run and also prevent it having the same fate as the original amp.
The current loom runs round the back of the boot and into the pit and I think it would be just long enough to enable you to take the feeds from the amp and put them into the headunit directly, and then remove the current wires from the headunit to the now redundant amp. It would be a bit of work but nothing that is beyond you I'm sure.
Actually you could leave the amp wires in the boot, take them to the firewall and you could fit crossovers there onto the wires directly and then run new wires from the headunit down the centre tunnel to these crossovers. That would keep it neat as well as giving you somewhere to put the crossovers.
The only thing I would suggest might be a slight hiccup is that the front right speaker is dual coil to allow the Bluetooth setup to work and thus you would make adding a new oem Bluetooth hard but I would suggest you would be more happy with a parrot or the like given the lack of nav screen anyway.