Any VPS running Unix with 2 GB of RAM & 40 GB or more hard disk space per Plone site will do the job for you.
Plone's min requirements are 256Mb Ram and 512Mb of space. You can go with that, but they recommend the above.
2GB of RAM VPS is fairy low entry system and would not cost you much. See more about companies who do VPS here: https://webhostinggeeks.com/vpshosting.html