A scope with parallax fixed at say 100 yards will cause errors in point of impact versus point of aim and/or blurry targets at shorter or longer distances. Here are a few links that can describe parallax better than I can.
https://www.snipercountry.com/scope-parallax/
https://www.fieldandstream.com/guns/what-is-scope-parallax/
An adjustable parallax, either side focus or adjustable objective, allow one to change the parallax for any given range which keep the reticle and target in focus for more precise shots. Now it is personal preference if you go with an adjustable objective or side focus on scopes.
As far as FFP and SFP, both have their positives and negatives.
With SFP scopes, the reticle stays the same size no matter if you are at the lowest or highest magnification. The negative to this is the reticle is only calibrated for one magnification setting, which is usually set to the maximum magnification. So the reticle will be off a little at other magnification levels. I'll use a MIL DOT reticle as an example. They are usually calibrated for the highest magnification. So on a 4-16 scope or a 6-24 scope, the reticle is only calibrated for either 12 or 24 power (depending on the scope magnification range).
The reticle on FFP scopes changes in size as you change magnification levels. So on th lowest setting , the reticle will be very small and on the highest magnification, the reticle will be at its largest. At max magnification you will loose the bottom portion of the reticle. But the patine here is that the reticle is true with POA/POA at any magnification level.
As fares brand or model, I'l let others give you suggestions. I will say that I personally prefer a MIL DOT or Christmas tree reticle for precision target shooting. Christmas tree reticles are "too busy" for some people and that is okay. But for calling wind holdovers, they do come in handy.