Servers become obsolete a whole lot faster than that.
In the U.S., you can depreciate it over a small number of years, or you can (depending on your individual situation and other deductions) most likely write it off completely in the first year as a Section 179 Expense.
Probably the worst thing you can do in your situation. Your bottleneck is bandwidth, not server horsepower. The least likely place you are going to get the needed speed and quality of bandwidth at a reasonable cost is in your home.
I'd guess that buying your own server will satisfy none of your goals. Assuming you are not going to have the server transcode the video on the fly, the server resources needed for this are minimal. The server is not the limiting factor.
I doubt very much you need a dedicated server. I think a VPS would be fine for this. Make sure you get one where you get a guaranteed percentage of CPU, and ask what CPU chip and speed they are using, and how many VPSs are on a computer.
Even if you did need a dedicated server, you are much better off leasing one from a host than buying your own. It will be professionally-managed, have backup power, and be located in a data center where suitable bandwidth will (hopefully...) be available. And you will have no obsolescence problem.
The total running time of all your films is irrelevant. The number of simultaneous viewers, required bandwidth, and available bandwidth are what matters.
It will cost you significantly more to host this in your home than to use a host. Not because of the electricity bill, but because of the cost of bringing suitable bandwidth into your home.
We can't answer that question. You can. Encoding rate x number of viewers.
But you don't just need "n" bandwidth. You need "n" bandwidth that can actually reach your users. You need to be hosted in a data center that has multiple large pipes to multiple backbone carriers, so that a reasonably-optimal path is available to most users. (And your host has to actually USE them... some hosts can be cagey about this.)
There are also services that will push your videos to "the edge" of the network using caching servers in multiple locations. For something like a high-quality Spiderman trailer, this is how they would do it. I don't know if this makes either practical or economic sense for you, though. Companies that do this include Akami, Digital Island, Mirror Image, Sandpiper Networks.