Hi,

Wind is wind and the biggest problem on the flat, but hills are worse.

Personally I think regarding brakes a little information would help.
A coaster is fine on the rear but a front brake will stop you far faster,
and for safety a bike should have two independent brake systems.

Its almost impossible to go over the front of a 20" shopper style bike.
(Unless you apply the front brake hard downhill stood out of the saddle.)

A single speed is probably no fun at all for those of advanced years.
On the flat there is still the wind to contend with, and it would be
no fun riding along with another geared bike with different natural
speeds relating to the conditions.

Gears are also needed to learn to spin, good for old knees.

Whilst some would say put the seat low etc for confidence, IMO
its very bad advice for old knees, and one good reason for the
crankforward type if the seat must be set low.

On a normal bike a low seat is a bad idea all round.

Better though to learn the correct mounting / dismounting style
of starting / stopping which doesn't involve reaching the ground
from the saddle, is easier on the knees and generally much safer.

rgds, sreten.