Put up a PadHerder and link it. Urd and Verdandi might put out decent damage, but their actives are going to work against you. It's quite possible you have some good utility subs sitting that can replace at least 1 of them.
As for number of stones, it's up to you to budget honestly. If you're going to IAP, the PCGF is a time to do it. They're going to be offering a 1 time bundle of 5 for $1 starting tomorrow, so that'll give you some good stones to start with. When I IAPed, I usually never spent more than $20, but that's just what I was personally comfortable spending.
I need access to a wifi spot to get it set up. I have an account but manually entering everything isn't an option while at work. I'll have it up when I get home though.
$20 sounds reasonable between what I have stored up in stones plus the 5 for 1 I should be able to hit that and get at least 5 pulls.