4) Ball speed at lower level - Contrary to popular belief, the highest speed of a 5.0 ground stroke isn't that different than the highest speed of a 3.5 ground stroke. The deal is that the AVERAGE speed of a 5.0 ground stroke is much higher than the AVERAGE speed of a 3.5 ground stroke. When you are not returning heavy ground strokes consistently, it makes it harder on the poly to do it's job as intended. On a softer shot, the ball just doesn't move the strings on the racquet face like a harder shot will. I will say that stringing poly at lower tension can offset this point as well as the new SPIN, ESP type of frames as it allows the strings to move more. Modern technology works in this case!!!
I think you have good intentions here, but the ball difference between a 3.5 and a 5.0 is huge. I've yet to meet a 3.5 with a true 100mph forehand... And I know several 4.5's and 5.0's over the years with 90-100mph forehands...
Also poly doesn't get hurt by not hitting hard...
Simply put, at a 3.5 level you really wont need the benefits poly has, which is spin and control. You're not going to have the form to strike the ball hard enough. So you're just left with poly drawbacks which are lack of power and lack of comfort.
It's best to wait until you really need polyester. And when you start hitting out all the time, that is NOT the time to switch to polyester.
Also someone posted about getting VS natural gut. If you want you can, but honestly at the 3.5 level you do not need to spend 40$+ on strings alone, for a lot of people its going to be $60 for a bed of it (with stringing labor).
Stick to multi-filaments. They have great feel and good power. They won't hurt your arm, and learning to feel what the ball does can help you get better.
NRG2 (about 16$ a set of strings) is probably my favorite multi and it feels very similar to natural gut when new. You can string it nice and tight and not run into arm problems. There are also plenty of 'mono core' synthetic guts that are very affordable (5$ a set of strings).
If you find yourself moving out of the multi range, try a cheap multi filament hybrid, like OG Gosen in the mains, and a cheap soft smooth poly in the cross. That will still be much more powerful and much more comfortable than a full poly bed. But still have exceptional performance. Better than only multi, and I find better than most full poly. Or even give Babolat Origin a try. It's a little stiff as far as synthetics go, but the energy return is very good, and it has great spin potential for a multi.
If you don't absolutely need the performance of a poly to play, you're much better off without it.