I'd say have at least 4GBs dedicated to the heap and optimize your Garbage Collection arguments as you'll frequently GC, causing server tick lag. The CPU is really the heavy hitter in Minecraft servers. Optimizing and removing GC lag gives more headroom for your CPU to process entities and tick cycles.
As clovux said above, we are suggesting the amount of ram that we believe you need from our past experiences. We are hosting companies that post in hosting discussion to help benefit the people who are having issues with their service, even though they might use us as a hosting company.
As for the disks you will want to avoid hosts using HDD's vs a host using SSD's
When we started hosting servers back in 2010 our first system was using HDD's in raid 10 which when it go to 24gb worth of customers on it the IO went high and could see issues with the drives being the bottle neck.
We then bought another server using SSD's and haven't used a HDD since. SSD's make a huge difference in performance.
You should be fine with around 2gb I'd suggest if you plan on running multiple worlds.
No host can say you will not have any lag as it's Minecraft and depending on plugins etc or location you could run into issues.
If you spend a little extra most likely boxes will have less clients on them as you don't need to crap them full for a profit.