Taxistaxing, I am not yet certain I want to fly commercially. However, I am planning my flying on the basis that I will do the CPL to be on the safe side.
I am not sure what you are saying in your post - are you saying that I should learn in an aircraft that is similar to what I will fly for the CPL so that I am used to it and therefore find it easier to fly, or that I should find an aircraft that is more difficult to fly therefore preparing me better for the more demanding challenges during CPL training? If it is the latter I still maintain flying something like a Cirrus over a PA28 or similar is ideal, considering it is more difficult to fly and demands a higher level of accuracy?
Unless your point is that I'd simply be tempted to just use autopilot and GPS.
It's not that the Cirrus is necessarily more difficult to fly, more than you'll presumably have learned with steam instruments, and will be faced with steam instruments in your CPL test, so 100 hours of 'glass', sidestick and much higher speeds in between might make the transition back tricky.
I went up with one of the CPL instructors at my local school shortly before starting my hour building for a few pointers and actually 100 hours isn't that much time when you consider what is required. It's very much 'back to basics' flying. His suggestions were: stay local; practise diversions until you can go up without a plog and 'divert' every leg of you route; and practise circuits and general flying being really brutal with yourself about maintaining headings, accurate height in the circuit etc. That seems to have been born out by the advice posted on this thread.
Also - yes you can switch the GPS off (although I expect the Cirrus system is very integrated into the avionics so are you sure it's that straightforward?). Just don't underestimate the "comfort blanket" effect of having a GPS in the aircraft. If you have nothing but the very basics it actually improves your situational awareness and you'll spend more time looking out of the aircraft (which is what DR navigation is supposed to be about).
At my flying club, on group flyout days, I'm the only one getting out my whizzwheel and chart rather than an iPad (I get a lot of stick for it too, and have been accused of being dangerous for not carrying a GPS 
). I've never yet busted air space or got lost. On one flyout I was bawled at by a pilot staring at his GPS convinced I was about to bust the stansted zone (I knew I wasn't because I was reading my CAA chart and knew exactly where I was). Not trying to turn this thread into a GPS versus DR slagging match, but you get the point.
I would consider speaking to the school you will do your CPL at and get their input. I can well understand the desire to fly a Cirrus (and you're quite right it's your money and no-one else's business what you do with it), but just make sure that you practise for your CPL in the most useful fashion. After all you aren't losing anything if you upgrade afterwards.
All the best with it.