In my personal experience, its better to do the right solution right away rather then do a quick fix and then run into problems later on. If you are actively planning to grow the network, I can only recommend doing the right thing now. Even if you already have 15 sites, its easier now then when there is 50 sites and you run into a problem with all the NATing, or you need to implement something new etc.
In a perfect world, all the networks should use DNS inside, so all you would need to do is renumber the hosts, (if you are using static dhcp leases that's easy) and change the records on the DNS server. Of course, I don't know you networks
Another thing to consider is IPv6. I don't know about your country, but we have some problems getting public IPs already now. Doing the standardized GRE/OSPF deployment would put you a long way towards being ready with your infrastructure when IPv6 hits. Just assign IPv6 addresses, config OSPFv3 and everything will work the same.
Either way, both of these solutions will work for your problem, the debate has turned into scalability and future growth
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