My neck of the woods... Live-no-more, CA
OK - I've done something very similiar to this... my main concerns was putting the water in the garage. Get's kinda cold in the winter & hot in the summer here, so I decided to place my holding tank next to the aquarium, indoors. Makes it very easy to test & adjust the holding tank water. However, I almost put it in the garage - figured I could just wrap it with a hot water heater insulating blanket and this would keep the temp a lot easier to control... but decided against that due to the occational 100+ summer temps out here.
Just a few notes that I had to learn about the hard way.....
1) be sure to add a pump in the holding tank to circulate the water (and check it every so often to insure it's working correctly) - or an airstone.
2) never let the heater in the holding tank get un-immersed (already lost one heater this way - worked OK for about 2 moths later, then died)
3) use some good pumps (I'm using the Eheim 1250's - the smaller ones just take too long)
4) use check valves to prevent back-flushing (orchard supply has some good ones)
5) use an immesable pump inside the holding tank to pump water into the main tank - an inline pump may loose it's prime after 2 days of non-use (mine did
6) Keep a floating thermometer in the holding tank to verify water temp.
On project 2 - I did something very similar as well - but I decided that since vacuming the gravel is going to always have a lot of particulate matter - I just use a 5 gallon bucket and do the vacuming the traditional way. After that - I have another pump to drain the remainder of the tank to the desired level with the outflow piped outside, dumping into the flower bed in the back yard. It's not as fancy, but gets the job done and recycles the water.
Good luck with the project - I find my setup makes changing out the water a breese.