-Clive is right. Additional braces are needed at Z. Take a look of build n1 in my signature, i am sure you have seen it but look again at the Z. Its just a 1-2kg more

-I would not bother so much for the 90kg.
First of all the so called speed you need at home is a fiction, so i would happily make a reduction if i am concerned with gantry weight.
I would suggest also for the price you are to spend on the closed loop servos to take a look also some servos. Just wrote about that in my build log #182
Just have in mind they are not tested yet. The Chinese i mean. Price is great though and it took me a lot of time to find them, not to speak about the emails.

-i like your gantry sides. I had to cut a windows on mine and decided exactly the same as you have drawn it will be better. Good that my girfriend tightened the bolts as i could not fit my hand

I just love this trend of "real" DIY machines. God, how i hated the crap especially in forums.