1. The ports are open - good.
2. Gettings the router with right firmware might be problematic? I think that they probably issue hardware with the latest firmware. Is the firmware downgrade possible? If I get the router with latest firmware and I don't get full admin access can I still configure the DMZ workaround that you describe in question 3?
Yes , you can set DMZ in the limited-enduser featured website that vodafone give access to you.
Downgrade is possible by doing a hardreset(connecting the power and pressing the reset hole by 20 seconds) and it will boot in firmware failure mode, then upload the firmware image that you wish.
Then use the bug and download de config edit to disable remote autoconfig of vodafone (TRC69 and SNMP) upload and they can't access and change things on your router.
3. Do I understand you right:
Vodafone ADSL Router ext: public dynamic IP <--- Yes public dyanmic IP, not shared or proxied
Vodafone ADSL Router int:192.168.0.1<--- It could be 192.168.0.1 but you can change.
Define 192.168.0.2 as DMZ host, disable dhcp, wifi, etc <--- Yes , all packages that not are in nat table must go to dmz, then also remove all nat/firewall table, you can disable firewall mode also in admin wan zone(If you need it).
My router ext: 192.168.0.2 <---Yes, and configure this as wan , not PPP interface only as switch or bridge without PPP.
My router int: 192.168.1.1
Are all incoming connections forwarded to my router external interface 192.168.0.2? So that I can forward them further to my "servers"?
¿Why do you want to use your router with WAN?
Connect it as simple ethernet switch enter on your router config set gateway and dns server the vodafone router ip and disable firewall and nat in your router , enable on vodafone router .... it's easier your wifi-N-gigabit router will act as switch gigabit switch on ports that it can and Megabit switch on the port that goes to vodafone router and it will act as wifi to ethernet bridge too (if no configure a bridge virtual with wifi and ethernet on your linksys in mapping zone).
Sorry for my basic english