Hey looks great, looks like my plan.
My plan is to have the whole system in line.
I changed my sump to the other end due to a buried sewer line on my property.
This became a bonus as I didn't need to pump the water as far and saved on pump size and plumbing from sump to fish tank. And for you from catchment to sump.
Actually looking at your diagram again, your catchment to sump is bad, you have constant height fish tank and when you pump water out of your sump your float valve will open topping up your sump, then your beds will drain into sump and overflow it.
In this system I don't think you can automate water height(

unless you put the float valve low in the sump), you just need to keep an eye on your levels and add when needed.
Only hassle with my new plan is the gravity return line from the grow beds is in the way, ( I have about 1in 8 slope from my fish tank to grow bed end) but it's not insurmountable.
I plan to have my sump right under the point your swirl filter is, with a bit of decking over it, plan to have radial flow filter on that deck.
On to your questions
Don't start with too many fish ( better to get it right on a small level and build up from success than to get it wrong on a large level and loose the drive to carry on)
Pump size? ( That's the bionic question) round byap it seems, the bigger the better, looks to me you are about a 1000 liter fish tank so you want minimum fifteen hundred Ltrs an hour at aprox 2 meters head, there really is no need to change your water through the grow beds that quick (3 or 4 times an hour) twice an hour should be plenty.
Other wise the math says 1000 ltr x 4/hr + 1000 ltr spare (extra can't hurt) = 5000 ltr /hr pump double that for head, and your looking 10,000 ltr/hr pump. (Seems head removes volume very quickly)
Don't know if my figures are correct just figure it's close to ball park. Help from experienced players please.
I have purchased a 4000 ltr/ hour pump, only plan on timing it 20 minutes on 40 off this gives my me a 1300 ltr./ hr flow. Probably on the minimum side but I'm on a fairly tight budget I think the pump cost me AU$80 eBay and runs at 25 watts (don't quote )
A lot of people here have different opinions on what a "good" pump means. To me, even on a budget, I would rather spend a bit more to get a pump that has a lower power usage and lasts maybe 2 to 5 years than spend less money and have a power hungry pump that lasts the same time. Or less power use replaced yearly if the electricity to replacement makes sense. I guess I'm trying to say, don't forget running / replacement costs in figuring the pump you need.
Some one here mentioned they use two pumps with less power use than one big enough to run the same amount of water. That got me thinking of running several small pumps on staggered system but that's a whole new post.
Any way good luck with your system beside that float valve, it looks good, don't forget when you start, take lots of build pics and start a system thread