Think iMac. Works well for me.

A G4 is going to be slow. Unless you get one of the higher rated ones in the 1.6GHz range but then you'll end up paying too much for too little machine.

For the price of a Used G4 powermac, you could get a NEW Mac Mini and use your existing monitor.(you may need to get a new keyboard and mouse.) Or, if you're patient and discerning(look out for scams) you may be able to find a decent used G5 Tower for a reasonable price. I recently stumbled on a dual processor G5 at 2.0 GHz 4gigs ram, 160Gig HD for 900cdn. Which is pretty nice for a reasonably potent machine.

I like Reason, I use it almost daily. For me it was worth the money.

As far as a control interface, I am a little biased, I don't really buy into this brand is better than others thing.

If you think about it is just encoders and switches, IE all digital signals that have a very slow input source...a Human. It’s not like analog pre-amp, the digital switch and pots don't give a unique sound, they just tell the software what button to press or what pot to turn.

SO...long story short anything will work, if it was designed well with the software you use.

I had the Mackie controller I liked it for reason but found it slow in Nuendo, likely Nuendos fault. Regardless I have seen the Ber-hinger mixer work just as well at 1/5 the price. So figure what configuration you want and try to cheapest box. If it works well for you great!

this is something else thats out there that i haven't tried but i wish i could.
http://proaudiomatrix.com/the-apogee/