If you're REALLY set on getting a DSLR, then here are my base pointers...
Use a kit lens or whatever for just walkaround general photography for now until you get enough to buy more EOS lenses, but for video applications, just go with some vintage glass. Pick up a 20mm, a 28mm, and a 50mm. (crop-factor will make these focal lengths more like a 35, a 43, and an 80mm). When you get the money for one, pick up a rokinon fisheye for around 300ish, if you really want that look.
With an adapter, it shouldnt cost you all too much for the three, and they will cover most of the same range as your kit lens, but will be sharper and, most likely faster - not to mention easier to dial in the focus, as the throw on vintage lenses tends to be around 180 degrees rather than the laughably short manual throw on modern AF lenses.
For vintage glass on a canon, probably go for pentax or nikon glass, since it will be the easiest to find and afford right now - the adapter is pretty cheap as well.
For a Nikon, I'm not entirely sure how vintage glass will work on a 3100 or 5100, but it should fit that mount at least, and be useable - which is all you will give a shit about anyways.
Another cost-saving idea would be to get an m42 adapter (or one for each and every lens you have, as i'd recommend - so you don't have to take them off) and invest in some older screwmount glass. There's tons of old Chinon, Pentax, and Russian made m42 glass available out there for really affordable prices.
...for any more info on vintage glass, just searchbar the official vintage glass thread.
