> 1) 3 10 GbE cards are maybe $400 each so does not sound like a very big investment. If you'd go with an external storage you'll end with a 10 GbE to get comparable performance either way
Yeah that sounds about right. And yes, 10GbE is a must as I see it, if using an external storage.
> 2) Backups should be physically separated from a production environment. That's for sure!
Yes, of course. But as long as you're not talking unique data, only server functions such as AD, PS, Exchange transport servers etc, it's not a big deal if you run the VM from it's backup location during the day, then by night restore it to it's original location. Or you can always setup a temporary backup to backup the backup 
If it's not too much data at least. There's an old swedish saying that goes "neediness has no law". As long as you know what you're doing, everything's allowed.