All you can do is reduce risk, and if you have more funds, you can reduce risk to a greater degree. This question is in the territory of tradeoffs!
If I had unlimited disk, I’d certainly add up to 4G of swap before even thinking about it - it won’t do any harm. If I had unlimited funds, I’d have the maximum RAM I could get. But in my case, I don’t.
If you’re expecting your forum might grow, you should expect to increase the resources you need to run it, over time, rather than expecting to size the machinery once at the outset.
I haven’t yet increased the sizes of the machines I use for my forums, but I expect I will, maybe this year or maybe next.