Lots of good advice there. Comfy saddle! Wide tires with minimal knobs.
Tall handlebars -- I put BMX bars on a lot of bikes. (You'll need longer cables.)
Best setup for gears -- in my humble personal opinion -- is rear gears only. This can be internal hub gears, og a derailer. But avoid a front derailer!
In my experience with my own fleet of loaner bikes, and the bike repair I do, it is the front derailer that gets people in trouble.
Many stores now carry inexpensive bikes with rear gears only -- mostly in the cruiser style. Way to go.
That said, a single speed will serve you fine. Make sure the chain is not too tight. I saw I lot of single speed bikes this year with the chain much too tight. That makes the bike very hard to ride. (I blame them all on the 16 year old "technician" who assembles all the bikes at the Reno Wal-Mart. LOL) The chain will become tighter as dust gets into each link and takes up a tiny amount of space at each pin, so it needs to start out a little bit slack.
Again... Biggest problem for most normal folks is correct operation of a front derailer. Please do not take offense! The factories (merchandisers) are doing the public a grave disservice by stocking the stores with umpteen-gear bikes. It's a marketing stunt -- it looks like what Lance rides. But normal people have no need for brain surgery tools either.
Single gear is fine. But I like a few gears on the rear wheel for when you have to ride against a strong wind.
I put a detailed explanation of front derailer (mis)use on a thread some time ago. With schematics. Poke around and you'll find it.