Using Asterisk as ISDN-to-VoIP-Gateway

I have a FritzBox Fon WLAN 7050 DSL-Router at my home, which makes it possible to plug in a ISDN (or analog) telephone. Then, the device acts as a special VoIP-ISDN gateway (in both directions), meaning I can make VoIP phone calls with my normal, analog or ISDN phone. I theory, this is great because I got a VoIP flatrate for $10 per month, but in fact the bandwidth management of the router isn’t very good, so the sound quality is very bad, especially under high network load.
So, since I have a small home server running all day long, I thought about using it instead of the FritzBox to do exactly that job, and I read a lot about asterisk that makes me think it could fit my needs.
Unfortunately, I couldn’t make it to find any information concerning my case, so I try to ask here:
Is it possible to solve my case with asterisk (receiving incoming and causing outgoing VoIP- and normal calls with my isdn phone)?
What hardware do I need, is it especially possible to use cheap ISDN controllers like the FritzCard?
Where can I find further information?

Yes you can

You need an ISDN phone card for your PC and a Modem supported by Asterisk for POTS calls

The FritzCard PCI would be a ISDN phone card? And do I also need a modem if I externally also have a isdn connection, or do I need a second ISDN card in that case?

I suggest using the X100p or a TDM400 card

and, yes, you’ll need a line for the ISDN internet, and a line for ISDN phone

TDM400 comes with 4 slots…