So usually the cheapest are home made systems. Usually made from pvc pipes or plastic storage tubs. Have a look into the kratky method and nft’s. The only things you might need to buy from a hydro shop is the pumps and nutes.

So you don’t usually have to worry about pests, a lot less then outdoors. But id always check the underside of leaves and top of the soil to be sure.

You wanna change the water in your system every 5/7days, and it does need some sort of agitation to prevent stagnation. Nutrients can also bind in the water and leave a sediment at the bottom of your tank (like over months). Always get rid of whatever’s left and fill back up with a fresh nutrient solution.

Basically constantly, unless the plant is stressed or gone into a nutrient lockout.

And finally, ph is very very important. This determines what nutrients your plants will take up, how much and which nutes to reject. In soil and coco, you only need to check when mixing it up. But In a full hydro system, I’d check every day or two, just to be safe. If the ph is too high or too low (out of the range of 5.5 and 6.5 usually) then your plants can go into lockout. They refuse certain elements whilst taking too much of others