There is always a learning curve when you try something new. How steep is really different for everybody. But I always say start small first, get some experience under your belt before expanding. You'll save both time and money. I wouldn't try growing root crops in a tube like that. It's just not a good system design for root crops, especially potatoes. One potato can clog your whole tube. Even the onions unless your talking about green onions. there simply isn't enough root space for them. And they should never be sitting in water or they can easily rot.
Radishes grow quite fast, and can do so in a shallow space. However a tube like that wouldn't be very efficient. If you just replaced the round PVC tube with a open rain gutter, filled the trough (rain gutter) with growing media like coco fiber (line the bottom with plebes), you could have a solid row of produce in the same space instead of a few here and there. you could do the same with green onions. However potatoes, bulb onions, and beats would all need a deeper root area. I would say at least 15 inches deep. The down side is they are all inexpensive to buy at the store, so is it really worth it to fill a large trough with growing media? I have a design to grow root crops like potato's, that doesn't use much growing media at all, and you don't need to pull the plant to harvest the produce (another system I plan to build). It's a simple concept and bit more of a complicated build, but I can't really think of another economical way to grow those type of root crops. Here is a link to a small simple
system I through together to grow green onions and radishes with.
If I were to grow radishes again, I would probably use row's of rain gutters like I suggested earlier. I would probably use the same sand and vermiculite/Perlite mix for a growing media. However I would need to mount the rain gutter troughs on 2x4's to support the weight. That sand is heavy, but works well and is real cheap if you get it at the right place.
I also have a more complicated system design for growing green onions, but in a small footprint would give me plenty of green onions each week (40+). However I need to work on the germination rate make it as productive as it can be. I think I have a simple Idea to germinate them without much labor transplanting them into the rotating system, but haven't tried it yet.
With the corn the variety I planed on grows about 3 feet tall and should produce about three ears per plant, and I think said goes from seed to harvest in 6-8 weeks. I planed to have rows. As an Example: If I had 8 rows, and the plants spaced 2 feet apart, that's 6 plants in each 10 foot long row. I should be able to harvest one row a week, and replant it. That should be about 18 ears of fresh corn a week. I can at least double that production (harvesting two rows a week) with a separate prorogation area for starting seedling transplants. Again I just need to work out the best spacing, system setup, and root mass accommodations. During summer corn is fairly cheep here, but I can grow it year round in a greenhouse. Just a little trial and error, and some math to see how profitable/economical it would be.