Computer-router. What CPU to use?
 

VPN-Internet, 50 Mbit/s. How powerful processor I should use to build a router that will transfer this traffic to 3 computers with no lagging? What CPU will be enough?

Any modern machine could do that very easily. Are you talking about using older hardware?

I am planning to use some modern laptop for that purpose. Something like
Lenovo ThinkPad R400
Lenovo ThinkPad Edge 13

Even older computers can be used as routers without any problems, I've done it before, an older P3 worked fine.

Why use a laptop? By definition a router will need at least two interfaces, and wired would be best so you can uplink to a switch. I would use a small PC if space is a concern. As mentioned the CPU shouldn't be much an issue, but you what distro are you planning to use? A router on its own will use very little CPU, but if you start adding on functionality like proxy filtering that will require a more powerful processor.

Laptop because some people will sleep around that computer. Desktop PC is too loud.
And I will use PCMCIA network card to make them two in a laptop.

And I don't wanna use just a simple router because it's not that stable, it doesn't have enough functionality. Will probably install Debian stable on the machine.

+ it will be data server for LAN, NAT, DHCP, firewall. And maybe an FTP server (to the Internet). All in one.

I would never recommend using your firewall as a server to store data, think about it you are putting information right at your doorstep. If you really want to use a laptop hardware wise it would be fine, I would look at a stripped down distro like Smoothwall or Pfsense rather than Debian. Or if you really want to build from scratch to get all of the details on how things work don't add extra services like a public web server, file shares, etc.

Are Smoothwall and Pfsense already configured as fully-working firewalls?

Yes, you will still have some basic work to do but they are full firewall distro's.

Can I confugure Debian firewall just like SmoothWall is configured?

I am pretty sure the underlying firewall in Smoothwall is iptables, but I haven't used it in a while. Debian also uses iptables, so short answer is yes you can.

Longer story is that Smoothwall is a web GUI in fron of iptables which would make it easier to configure. Debian would be manually configuring using the cli.