When it comes to website hosting, your main concern should probably be the amount of RAM as I've found from experience, not having enough can lead to all sorts of annoying problems and crashes, slow performance and downtime.
You should be ok with any VPS server sporting 4GB of RAM or more, provided you're running linux. If they're just static WP sites with not much traffic, you could even get away with 2GB ram but you'll want room for expansion. Opt for SSD storage if possible as I don't imagine these sites to be too large, however you'll want to take into account if your clients are going to be using emails/webmail.
Most VPS providers have in-place instant upgrading, so it might be worthwhile starting off with 2GB and seeing if you need more at a later point in time.
I use MediaTemple and am hosting several large high-activity client sites without any hitches on 4GB RAM and 60GB SSD storage.
Another thing you'll want to take into account is location. For me, I'm with a US host but providing services to Australia. Having a datacentre on the West Coast was important and MT ticked all the boxes.