Ken,
trust me on this one as we bought our first house just 3 years ago. MOVE the LAUNDRY AREA INTO THE HOUSE. Get it out of YOUR GARAGE NOW!
Reasons:
#1 gives her no excuse to go into YOUR garage.
#2 Gives her no excuse not to do laundry.
#3 Gives her no excuse to store HER stuff in YOUR garage.
As for tank size, I too think you should go as big as possible. Just do it smartly and the cost won't be as bad. Things like eductors for flow on C/L don't add electrical cost. A few good size/low draw pumps for circulation. Light movers for light systems eliminates extra lights/cost.
I started with the idea of a 150 I already had. Wound up trading the lighst i had for it and a little cash for a 190 and sold the 150. before I could get it all built, found a deal of the century on the 400. Got it and sold my three tanks, 190, 135, 60 that were sitting in the garage waiting to be set up. Now me AND the misses are happy we got the 400.

take your time and look around, you'll find a good deal somewhere.
Oh and you'll have frags waiting for you when you big tank is set up & running.
