VPN, NAT - an out of the box solution

Question

I am looking for an out of the box VPN solution for homeuser build with cheap hardware. The szenario is as easy as everywhere:

Office:  
Server (private IPv4 LAN 192.168.10.x) <->  
router and VPN-server (LAN with NAT, WAN with fixed/official public IPv4 address) <->  
cable modem <->  
internet (IPv4)

Home office:
Client (private IPv4 LAN 192.168.11-99.x) <->
router and VPN-client ("thebox", LAN with NAT, WAN with private IPv4 assigned by internet-router via DHCP) <->
unknown natting internet access equipment which provides IPv4 addresses on LAN side by DHCP <->
internet (IPv4)

What I am looking for is "thebox", a cheap device which is able to create an VPN tunnel through the "unknown natting internet access equipment" to the "router and VPN-server" in the office which has an static public internet address.

I found e.g. the cisco RV130 or the TP-Link TL-R600VPN, they can be an L2TP client but without IPSec. And I did not find the counterpart, a cheap router that supports to be a L2TP/IPSec VPN-Server.

So I am looking for e.g. an VPN-Server that ca be a VPN server for 10 L2TP/IPSec clients and the counterpart VPN-client-box that can create an L2TP/IPSec tunnel on the WAN side (assigning WAN IP address via DHCP) through a natting internet access equipment.

All this in anticipation that L2TP/IPSec works at the client side through a natting router. If somebody knows better, what is the best VPN protocol which can pass a one side (client) natting?

We need a VPN-client-box which opens a LAN-tunnel to the office because there is no possibility to install any VPN client software on the home office client itself. Furthermore, there may be more than one client in the home office.

Thank you for any support in advance.