Ahh.... you did say it was free, correct?
Then what exactly is the issue?
Take the saw, use it for as long as you deem neccessary.
If it ends up being what you want - great, didnt cost you anything.
If it ends up NOT being what you want, youre a step ahead of everyone
else already - you found that out for free rather then spending the 500
Go for it.
All that aside, I think 10' is more than sufficient for _most_ people.
Unless you plan on cutting massive mouldings are large wood, you dont
need the 12' or a slider (both of which will take up more room).