paddle fishing kayaks--- some generalizations but....lower priced ones are cranked out fast leading to less uniform plastic thickness and faster cooling times (kayaks shrink after leaving the mold, if not careful different areas can shrink at different rates. Take measurements of the same model, they will vary on all brands
obviously some have greater variation). Lower end models tend to be thinner (nice when you carry it but not so much when you drag rocks).
The ability to stand up in kayak jumps you up to the upper priced models (designed consideration for wide flat area to stand but not too much width to slow the boat down). For me, I won't be with out an elevated seat even if I don't regularly stand in my kayaks (first 15 years I kayaked fish no kayaks had been designed yet to stand in
), although it is must have feature if you plan on flippin/pitchin jigs.
You mentioned river fishing....hull design is different for riverboats. Nice smooth transitions (ie rounded bottom) with minimal keel/rudder is what you want for current. Along with some rocker (banana shape bend from bow to stern). Rocker for quick turns in current and rounded edges for current to slip by (extra long keels catch too much current). Lower priced models tend to have excessively long keels for both stability and to make it easy to paddle straight, but the "super" keel also makes it difficult to turn. IMO it is much easier to learn good paddle strokes than depend on hull design.
As others have mention, demo as many as you can stand. The difference I talk about above, become more obvious the more you paddle. IMO their are only two major players in the kayak fishing (paddle) market, WS and Jackson. Jackson are made here in TN. There are several others that make quality fishing kayaks but more of niche fillers such as Feel Free, NuCanoe, Native and others.
Lastly be sure to leave some money in the budget to get a quality personal floatation device (pfd) that you will wear every time on the water and a quality to paddle (need to figure out if your a high-angle or low-angle paddler first).
Summary, yes there are differences between low end and high end models. You are the only one that can determine if the extra cost is worth it, but that is nearly impossible to do until you spend some time in the yaks. Good luck with your decision. More than likely, you won't be buying your last kayak, so don't fret the decision too much, as it is more important to just get out there and start enjoying it soon rather than later.
PS pedal kayaks Hobie and Native are the only game around.