Maybe I found an affordable alternative
Browsing the TP-link web site I found the TL-R600VPN.
I read specifications, but I'm not able to find how many connections is able to manage it. If I'm not reading the wrong parameter, the R600 is able to manage 10000 connections, and the 5120 can manage 120000 connections. If I use 200 or 300 connections, then the R600 should be more than enough. However, the R600 has lesser RAM, so which one is better for me?
PERFORMANCE
Concurrent Session 10000
NAT Throughput 120Mbps
IPsec VPN Throughput (3DES) 20Mbps
This is a VPN router do you need a VPN router. Get the Load balance ones instead. I am the 5120 and have a Smart Switch on it's way also here to replace two un-managed switches. 5120 has managed switch ports function.
You can get the 5120 very cheap today depends on where you get it from though. Some greedy vendors online charging way too much today.