I say, if you can't afford to do the whole whack at once, its possibly actually more fun to do it in stages.
Once I got into it, I loved doing the ripping the car apart and stuff, it's all good fun!
That being said though, there are some rules. For example, you shouldn't half-finish an individual stage while you wait for more money, i.e. fit new front speakers and have to wait to buy the amp for them. Do the whole lot in one go.
Same for sub/amp and other bits and bobs.
I did it a different way

I did the whole lot at once, and then 2 weeks later got bored so re-did half of it with some extra bits
Beware - ICE is a terminal disease (until your money runs out!)