Server<-->Client

1.Should I connect router to mac mini or I can make without it?

umm, how is your Mac Mini connecting if not through the router...?

In other words, I absolutely expect the router to be inline.

2.What services should I start on Server (dns, vpn, firewall)?

What services do you want? At the very least you need VPN (since that's what you're trying to run here), but the rest is up to you based on what you want/need the server to do.

For most purposes you're going to want/need DNS on your internal network. That could be on this machine or another machine on your LAN - your choice. Same for any other services.

3.I don't know what is starting IP and ending IP (in L2TP settings). Whee can I get it?

That's entirely up to you. The address range is just a range of IP addreses on your LAN reserved for VPN clients - when a VPN client connects it will be assigned one of these addresses.

You can choose any range of IP addresses that are valid for your LAN, and that don't conflict with other IP addreses (e.g. your server, printer, DHCP range, etc.)

For example, if your LAN uses the subnet 192.168.1.0/24, you might have your server at 192.168.1.1, a printer at 192.168.1.2 and your router at 192.168.1.254. You might then have a DHCP range of IP addresses from 192.168.1.3 through 192.168.1.100.

That leaves anything between 192.168.1.101 and 192.168.1.253 as spare, so you can choose a range of IPs in that block for your VPN client.

Obviously, the specifics will depend on your own network setup, so you're the only one who can answer the question.