Short answer is that it depends on a massive number of factors.
You can run a forum like this with hyper aggressive page caching on a slower / cheaper server, and have it work acceptably 'most' of the time, but glitch out under times of heavy load.
Your next challenge is backups.
Assuming the host has these or relying on RAID is lunacy (RMWeb which is somewhat larger than here & backed by a major publishing company to boot) had a major data loss incident c. 18 months ago.
Troubleshooting as noted above.
Anything this size will have script kiddies all over it in a rash, all poking for any hole they can find.
Plenty more I haven't though of off the top of my head.
Sure you can skimp on everything, but sooner or later it bites you hard....