The bent chainrings could be straightened maybe, or you can just go to your local bike co-op (a place where old bikes go to die at the hands of volunteers and which usually sells the second hand parts) and buy a second hand one. Replace the middle one, the only trick is making sure the mounting holes are the same pattern and distance apart (that's called the BCD, a bit like fitting a wheel to the car, you've got to have the holes lined up).

As mentioned, use the existing rear derailleur as a chain tensioner unless your bike has horizontal dropouts which allows the wheel to move backwards and forwards to tension the chain.

Keep your existing cassette of gears on the back until you work out which one you want to use all the time. Simply put the chain onto that cog and then adjust the derailleur so it doesn't pull the chain onto another cog. Or keep the rear shifter and change gears as you like.

A GOOD SS bike build will cost you a bit. My first SS build (actually, it was fixed gear but same difference) cost me a cog, a chain (because the old one was worn out) and some barked knuckles (because every good project needs to be christened in blood). Mind you, over the next three years I spent a lot of money on that bike improving things and getting it right, but the initial cost need only be minimal.

You were linked to Sheldon Brown's articles on SS conversions above.
I'd like to highlight Fixed Gear on the Cheap from the same website.