I'm costing sand and rock for my tank, which i'm in no rush to add fish to once i've added water. I'm considering adding a fair amount of base rock and letting it become live over time before adding fish, is this a particularly bad idea? i know it'll be months and months before i can add fish, but that's not really a problem.

I've wrked out i need about 30 - 40 kg of rock, since, apparently more is better. (the tank will probalby be about 65 gallons) so i was thinking about getting 10 or 15kg of live rock, and the rest as base rock. also thinking about getting 4 bags of base sand and two bags of live sand. are these mix levels a good idea, or should i go with more live, less base?

also, any guesses on how long it'll take to seed the rocks, etc?

actually this is a great way to go, its how i have done my tanks.

you can use mostly BSe rock, and some LR, let your tank cycle, and then you can start adding your animals, now here is the catch, you can add some, but not a lot, and not fast, because the base rock is not live (as you noted) you need to wait, but its not a show stopper, average is about 4-6 months before its actually doing something worth while.

the key is to move slow and observe as you go.. :)

I was thinking about leaving the tank for 2 - 3months before even thinking about adding a fis. would i need to feed the critters in the sand and rock during this time? and what should i feed them? do similar to the cycling and put a bit of shrimp in there every so often?

2-3 months is about right, as to feeding the critters that kind of depends on the hitchhikers you get with your LR and LS. I've been very lucky and gotten most of my corals that way :D and more then a couple hermits as well. Those can fend for themselves but periodically you get something.....strange those may need to be fed depending. Mostly you can just let it go to work, mother nature is awesome at taking care of her own.

You could add a bit now and then but I don't think I would worry very much about feeding. The critters that come on your rock and sand are usually tiny detrivores and will clean up the die off from your LR. (Even thought you are using mostly base rock you will still have die off on the LR.)