What a good question. You need support for 1,000 VPN tunnels (minimum 700 for "right now"). Only look at the "Recommended" count, not the maximum.
https://documentation.meraki.com/MX/MX_Sizing_Information/MX_Sizing_Principles#Number_of_VPN_Tunnels
If we take a look at the Meraki "Product End-of-Life (EOL) Policy":
https://meraki.cisco.com/meraki-support/policies/
It says that:
"Cisco Meraki typically provides support for a given product for a period of five (5) years after the EOS date."
EOS has not been announced for the MX250. So you could reasonably expect a minimum of 5 years of life left.
Given that, and making the assumption you are buying licences that are 5 years or less, I would personally go with an MX250. Also you are only using AutoVPN, so even if feature velocity slowed on the MX250 - you probably wouldn't be using any of those new features.
For the cost of an MX450, you could probably use an MX250 for 3 three years, throw it away, and replace it with something else (perhaps an MX106 will be powerfull enough by then) and still come out better off financially.