A four-input diseqc switch will let you select between LNBs.
But, wait 'till you have the Primestar to see if it needs a 2x2 multiswitch or not.
Really old ones might, but depending on what LNB is on it, it may not.
Or, you might swap out the LNB to bring it into the 20th century...
...but leave the feedhorn as-is.
As for the DTV dish, we would need more info as to which one you use.
The simple old dishes with separate LNBs are easy.
The later ones with some built-in switching may need some discussion.
The very latest big dishes, are another animal.
Lot of receivers ship with a complimentary 4x1 diseqc switch.
They can be had cheaply, if you need to purchase one.
They are a bit delicate, so be warned, turn off power to the receiver before connecting anything along the coax path!
As for your questions, they show you are thinking.
That'll help you get things rolling with minimal surprises at the last minute.
