POTS/ISDN lines and FreePBX

Dear folks,

I would likes to ask another question, and it seems sounder making separate post for this as it pertains to something else.

I apologize if my questions sound banal, but this is the first time I’m dealing with VoIP, and PBX systems in general.

Currently, our phone system is using 10 outgoing ISDN lines which we would like to continue using with FreePBX (i.e. we’d only like to use VoIP internally) and have outside calls go via those lines.

I understand I’d have to buy a special hardware card that takes those lines and makes them usable for FreePBX/Asterisk. But what kind of card/trunk would this be? There are so many different types! (IAX2, DAHDI, ZAP, etc).
Would the configuration be straight forward, or would the phone company need to do anything?

Also, for HA, I’ve tried the backup/restore. If I get another machine with the same card (whichever one I need), could I just swap them out and restore the backup and replug the lines in the new one?

I’ve read the HA FAQ, but has anyone made a system with automatic failover?