I'd suggest you get started with your proposed hardware. The plan will change as you experiment, learn and probably destroy components. No need to over think it at this stage.
On ebay they sell cheap weight sensors...you can put the plant in a pot and let arduino weigh it all the time and send the signal to you by wifi. So you can make charts for temperature and how much the plants drink/grow.
That is a nice idea, but i decided beforehand that i would only monitor parameters that I could alter. That basically means watering, ph value (nurishment) and the amount of light because my tomatoplants are all in pots so I could move them around.All the other stuff is neat and cool, but it does not let influence the tomatoproduction without extra cost. So i really try to minize technology where the benefits are minor in relation to how I manage my tomatogarden.When the time comes that I have a raspberryhub running, then it could be cool tho because I would have an interface in the room and a nice layout with some extra info. But till that time, I will minize sensors where I can.To add/edit: Just bought all the stuff above and 1 extra package of resistors/potentiometers/leds and some buttons for €2,--, it takes about 30 days to arrive according to aliexpress so i have time enough to read up.
Yes the costs can add-up fast. You could even build the pot on motorised wheels so they drive themselves to a place with the right amount of light/temperature in the garden.
Controlling ph is a hard one to tackle. Best is to mix good soil in advance. And tomato's like potassium, potassiumsulfate is a very good fertilizer for them.I have a potplantmeter here (cheap one) but that doens't measure ph properly, a good one costs 100 us$ or more. Even moisture it doesn't measure well that's why i was thinking about the weightsensors. With those you can see how much water evaporates every hour and compare it to temp and sunlight.
I grow many fruittree's in large pots (rare tropical ones) but watering is done by sprinklercomputer from Gardena. You can build one of those easy with a solenoid watervalve connected to the tap and arduino controlling the valve. Best results you'll get by a drippingsystem, arduino can fillup a watertank which drips slowly on the pots.