If you don't already have a super-huge traffic site, you probably won't have any problems here or anywhere else -- unless you did something to break the TOS like distribute pirated software, copyrighted material, etc.
That's impossible to say, because it really depends on what you're going to be doing with it.
Think of bandwidth & resources like years & mileage with a car warranty. If you have a 5 year/50,000 warranty, that warranty ends in 5 years, even if you only drove a mile. On the other hand, if you put 50,000 miles on in 2 years, you just shortened your 5 year warranty by 3 years.
Poorly coded scripts and database queries can cause problems with a high traffic site.
By itself, it's not a problem. You could have a slow bloated site with no traffic and not notice a problem... unless you get slashdotted/digged/etc. Some scripts also have caching options or plug-ins to lighten the load.
If you have something that isn't going to be regularly updated, but will get flooded with traffic, it wouldn't be a bad idea to make that part of the site static HTML.
Some people will do that if they have a single article that's getting hammered.
Like I said though, if you're new to hosting & don't have an established web site, you're probably not going to use even 1% of what's available. I'd guess that a very high percentage of DH users are using less than 1% of their available space & bandwidth.
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