The smart TVs cost more and do less than a TV and separate unit. Further, the smart part will become depreciated and eventually useless. The only advantage is having one remote to track, rather than two, but that's not a big problem with today's remotes that can handle the smart devices pretty well.
If the cost differ between a smart tv and a regular is under $80 then you might as well just because. Otherwise, it's not worth the cost increase.
LED tvs are actually LCD tvs with an LED backlight, which makes them consume less power, makes them lighter, should make them last longer, and in the higher end tvs improves contrast than non LED tvs which use special fluorescent tubes for the backlight. They are more expensive, though, and for generic TV watching the difference isn't often worth the cost. I personally wouldn't buy a plasma. They have better contrast, but are much heavier, have problems with burn in, and are going out of style. But they can usually be found cheaper, so it might still be worthwhile. They also have better viewing angles than cheap LCDs. If you move all the way around a cheap LCD you'll find that past a certain point the color or brightness of the image is bad, usually at very extreme viewing angles. Good LCDs have better viewing angles but if you LAN on watching TV from nearly edge on, then a plasma might be better.
Right now, roku is top dog. It can be had inexpensively, especially if you hit a sale or refurbished unit, and has all the major online viewing platforms that have subscriptions. There are a few that aren't due to exclusivity with apple, google, Sony, or others, but nearly all support Hulu, Netflix, and the other major platforms anyway, so it's only until you want to get other channels like college TV streams, Disney, and others where roku really picks up the pace. If you find yourself using an iOS device a lot to view entertainment, then an Apple TV is useful, otherwise it's not terribly interesting.
120Hz or 240Hz frame rates are useful if you've ever noticed jitter in other people's tvs when they watch blu ray movies. If you've never noticed, then chances are it isn't ever going to bother you, and you can ignore the higher frame rate tvs. What the deal is, if you're curious, is that the display frame rate is fixed. Most a 60Hz unless otherwise advertised. For normal TV viewing that's fine, since its a multiple of broadcast TV frame rate of 30frames per second.
However movies are 24 frames per second, so you end up doubling some frames three times and others two times in order to map 24 frames per second to a 60 Hz display. This odd beat sometimes can be noticeable, particularly in panned movie scenes. By getting a 120Hz display it can properly display each frame for the exact same amount of time, leading to a much smoother presentation.
Beyond that, check out al contrast online. Many have video processors that change the video in order to give enhanced, but not real, contrast. It's better to get a display that simply has good contrast than one that fakes it by fudging with the video.
All that being said, you'll probably be happy with whatever you get, and you don't need to fret too much over your choices. Even today's bad tvs are leaps and bounds better than tvs five years ago.