Is this setup feasible for a power efficient 25G router?

Hey folks,

I started a planning a small home project where I wanted to build an energy efficient 25G capable router... Not sure if this is a contradiction but I thought, let's try. I need your advice though.

Here's one potential setup:

• Fujitsu D3642-B
• Intel Core i5-9600T
• Mellanox ConnectX®-5 (10/25G dual ports)
• Quad 2.5G ethernet NIC (to be decided which one)
• 32 GB RAM (to be decided which one)
• 1 TB SSD (to be decided which one)

The network would roughly look as follows:

• 25G WAN link
• 10G backbone to another switch (routed to the office where my main workstation is located)
• 2.5G Ethernet will be routed to APs capable of WiFi 6

And the setup should then enable to:

• Saturate the 25G link during peak times
• Be as power efficient as possible
• Run a hypervisor (preferably Proxmox), where OPNsense would be virtualised

I haven't built a PC in over 20 years and I'm not able to find all the info I need. Would you guys be able to help me please? For instance:

1. How can I find out if the PCI-E slots of the mainboard are directly connected to the CPU to avoid bottlenecks, especially when I want to saturate the 25G connection (linking to then other 10G slot as well as the quad 2.5G cards)
2. Do you guys have a recommendation for a good and affordable 2.5G card?
3. Is the processor powerful enough or maybe even too powerful?
4. Do you think the setup would actually be power efficient? I'm looking at something around 20W on average but I have no clue if this is feasible at all.

I'd really appreciate any help or pointers!