2GB RAM should be enough for what you're looking to run.
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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.
The amount of RAM I recommended was from my own personal experience when managing a server similar to what you're looking to run. It's always best to start out with a smaller amount of RAM, as you most likely won't need a huge amount in the beginning.
Also, almost all hosts allow automatic upgrading/downgrading of plans through their billing panels, so if you do need more RAM, this can be done easily.
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.
You'll be fine with a low amount of ram as long it's not oversold by the host which is probably is when you see prices like $1-2/GB, the most important thing about the RAM is that you need to have enough of it so that you're not trying to use more than the host can provide, that would cause a stack overflow and your server might crash. Another important aspect is the CPU as Minecraft is pretty CPU heavy too, as soon the CPU load goes up to 100% you'll notice lag instantly so make sure you'll check that as well instead of staring yourself blind on the price per GB of RAM like some people do sadly.
Last but not least, the disk, is it a regular HDD or a SSD, regular HDD's on a shared host could be busy and thus cause lag by randomly freezing for a few milliseconds/seconds at a time. Lack of bandwidth could be a reason for lag too but I guess it's not that common after all.
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.