Before you even start, make sure you've got horizontal dropouts, a frame with vertical dropouts isn't worth converting in my opinion.

To answer your questions:

1: With horizontal dropouts you won't need a chain tensioner, but you may need to get a half-link to get the right chain length.

2: You're better off with a single speed wheel, things will get wonky with the bottom bracket and chainline if you ever want to upgrade in the future. Be sure to check the spacing between the rear dropouts on the frame, I may be wrong but I think most single speed hubs are 120mm now.


And yes, Sheldon Brown has everything you will ever need to know in life.