What software is running now? The first step, I assume, is migrating the data.
How many posts are there? How big is the current database?
How many pageviews do you have and expect? The number of users matters little, the number of simultaneous users matters much more.
Knowing nothing about your traffic, I would guess that a 2 to 4gb EC2 would suffice. For a frame of reference, on digital ocean that’s US$14-28/month.