i have 2 30 gal. recycle trash cans that i use for mixing. i have them next to the deep sink across the basement from my sump. i am curious as to how you are planning to plumb your external pump to do the jobs you have outlined.
i use $15 wave pumps to do the mixing and transfer. you don't say how big your aquarium system is. mine is 100 gal total.
I turn on RODI which fills the FW can. this has a pump in the bottom which mixes the Prime that i put in it. this same pump with a short hose attached to output pumps the FW to the SW can next to it which also has a pump in it plus a small heater. the salt is mixed in this drum and the pump is strong enough to keep the water moving until the salt is dissolved. the transfer is slow if i use the wave pump to do it but i have a Sicce Syncra 3.0 that works very well. i have it on another system that has a 4 ft lift with 3 90's using 3/4 " tubing.
https://media.cdn.bulkreefsupply.com/media/wysiwyg/PDFs/syncra.pdf about $100