How much smaller frame is ok? I'm looking to buy my first MTB and I had an online calculator tell me to get a 17.5". My measurements are height: 177cm and leg 76cm.

Thing is, I found a bike that I REALLY want, but it's the last one and it's a 15.5" hence the good price. If I don't get this particular one I'll have to get something significantly lower spec as this is massively discounted.

I don't intend on doing really long rides ever. Maybe 2 hours max.

1.5" difference doesn't seem that much.. Will this be ok or am I about to waste allot of money?
You can use a bike that's "too small" or "too big" without too many problems, so long as you set it up properly for your own biomechanics. 

Try using this fit calculator instead: http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO

Compare the bike you really want with the results of the calculator. It will give three separate options, and provide you with a rough idea of whether the bike is likely to be a reasonable fit for you. 

1.5" doesn't sound like a lot, but in the world of bike fit it can make a massive difference. Ideally you want to work around things that don't fit by changing components, but you need to know first if changing the components will help.