watch glen's video for one if you haven't already. Then I'd recommend watching one for how fish finders actually work and what they are good for showing (and what they aren't good for showing).
A couple basics:
- A total fish finder setup means a head unit, a transducer, a mount, and a battery.
- Live imaging you can be sitting still for. For basically all other bass fishing purposes you need to be moving for the fish finder to give you any type of useful image. Yes it will display an image, but its just taking repeated snapshots of what's directly below you and not a scan of the surrounding area. So keep that in mind- for your fish finder to be useful it means using it while you're paddling along ideally at a constant-ish speed.
- Mega live (and live scope and panoptix) are transducers. They are live imaging transducers. Terms like mega SI/DI, triple shot, split shot, and 3-in-1 are the common names for the non live imaging transducers from the various manufacturer.
- what imaging type you get will depend on what you buy. Since you mentioned ML2 I'll use Humminbird terminology (with which I am familiar). Side imaging (SI), Down imaging (DI), 2D/Chirp, and GPS are the main features in the head units as you would get them out of a box (let's leave out 360 and Live for a minute). HBird starts with Chirp + GPS. The next status up is Di which includes Chirp and GPS. Then SI includes Di, Chirp, and GPS. SI is the most fully features model. You will also see reference to Mega and '+' in the descriptions which are references to the higher frequencies available on those models and which are depenent on the transducer and not as much the head unit. If you buy a SI unit, then you will have all of the other views available as well.
- Mega Live and 360 Imaging are add on transducers. In theory you could buy a control head only (no transducer) which is Si capable and add on ML/360 and in that case you would NOT have side/down imaging though your unit would be capable. You just wouldn't have the transducer for it. In practice, so long as the head unit you buy comes with a transducer then that won't happen.
Now to your specifics.
What do you want to do with the fish finder? what are you expecting it to do for your fishing? Those answers will inform what you need. I'm going to say that you almost certainly don't need to spend $5k and you most probably don't need 2 head units.
For a kayak, the 7-9" size is a pretty big unit but still decently manageable. As you start to get bigger you start to need specialized mounts and the total weight of the head unit gets to be a lot. Even going from 7" to 9" is a huge jump in size. Bigger screens are nice if you're trying to split it 3 ways and still have a big SI or Live image. If you're using live imaging and trying to scan out to 75' in 30' of water while watching a 3" bait then a big screen would be nice. Outside of those scenarios, a 9" screen can do a ton and be very clear.
You asked about vegetation. You're in CT and your waters are a lot like mine. I'm going to say that the answer to your question is 'no'- they aren't going to pick up bass in vegetation the way I think you're thinking about it. I might be interpreting you wrong, but if you have a grass bed on a big flat you're asking if it will show you were in that grass bed the fish are. The answer is no. Think of the transducer as a flashlight. From where you're sitting right now, look around the room and pick out a couple objects between you and the wall. Now shine your flashlight at the wall. The objects are clumps of grass. Do the objects block the light from shining behind them on the wall? What if there was a fish behind that object, would it be lit up with the flashlight? That's how sonar works. The beams get blocked by whatever they hit. They can't see behind objects. If the fish is in front of the grass in the open then it might be able to pick it out. If its inside the edge of the grass, then depending on how thick the grass is you probably won't see the fish. Below are two examples. the top is a split screen of down/chirp/side imaging. The second is live from the same place. That grass is 5-6' from bottom to top. Notice you can't see 'inside' of the grass in any of the images (yes, the 2D is 'hot' and could be better). There are certainly fish in there (I caught some skimming the top of it).
If the grass is thinner, then you might be able to pick out fish. The image below is a pretty heavy crop from a bigger picture, but you can see the sparseness of the grass in the down imaging (blue) on the left of the image and you might be able to pick up fish in that. The heavier grass on the right (which you can see in the SI on top) you couldn't pick one out. So relying on your FF to pick up fish IN grass isn't something I would do.