I know the 50/200 + 455/800 HDI is for deeper water but will there be any disadvantages of using it in shallow water?
I ask because i can get it cheaper than the 83/200 + 455/800 HDI about $50 less.
Should i spend the extra?

The 0 kHz is so wide it is of no use unless you fish the ocean or troll with planer boards. If you only have one unit it won't make any difference since you can use 200 on either.

That is odd on the pricing, are these new transducers or used? All of the pricing I have seen shows the 50/200 to be about twice as expensive as the 83/200. $215 versus $110. By using the 50/200 instead of the 83/200 you will get reduced target separation. The 50kHz will give you more bottom coverage, but because the cone angle is larger, the definition in the image is not as good as what you will see with the 83kHz. You could select the 200khz setting on the 50/200 but the 50/200 has a narrower beam angle (12 degrees) on the 200 setting than the 83/200 (20 degrees) does. The 200khz would give you better definition but with much less bottom coverage. Another thing to consider is the size of the transducer. I haven't seen the ones for the HDI but the 50/200 is normally larger than the 83/200. Is this unit to be used at the console or the bow?

I will be using the down scain 90% of the time and the deepest water i will see is 100' but mostly less than 50'.
As it will be on the trolling motor.

If not to late do not get the 50/200. There is currently no trolling motor mount for it. It is huge in comparison to the 83/200