Sounds like you're planning and that's the good thing! You came to the right place to get an idea and tips to make your plan work out! Welcome to the forum!
For your sump questions, I have no recommendations since I haven't messed with them (yet!), but I'll be sure to follow along this thread to pick up some tips as well.
Tank size really depends on your overall goals. I'd suggest getting a little taller tank for your discus, but too tall and it'll be harder to get enough light to some plants (if you were to want a carpeted tank you'd need a stronger light or pair of lights to ensure there is enough to promote growth). I have Fluval 3.0s and they are freaking awesome but expensive.
As for water parameters go, in my honest opinion, consistency is more key than hitting a certain number. Both of my discus tanks (125 gallon domestic and 75 gallon wild) both survive and thrive on my tap water (treated or aged of course). My water is slightly hard but pH is around 7.4 to 7.8. Depending on the size of the discus you purchase, regular water changes are absolutely a must for optimal growth so I'm weary when new discus keepers plan to "minimize maintenance" because water quality is everything to discus keeping.
As for stocking, I haven't kept loaches with my discus, but I'm sure it can be done though I wouldn't add any snails as your loaches will just eat them in my experience with my other tanks (I have 11 operating and a few on stand by). I keep corydoras, the sterbias and the albino corydoras seem to fair better than other types in warmer water. SAEs are awesome to get rid of algae, I love them and they even swim in schools with my rummynose tetras which is awesome to watch.
Since you are planning on planting, make sure you are choosing plants that will be fine in your sand substrate, you may have to consider root fertilizer tabs and other liquid fertilizers to help them thrive. Also note, with dark substrate or backgrounds, your discus can lead to peppering if they are pigeons which is a turn off for some. I have a sand substrate and I have a dwarf sag carpet growing. With a carpet, you will have to syphon vac consistently because the carpet of dwarf sag gets thick and will trap in debris. Corydoras and wave makers can help, but they are not the answer like a good vac is. So skip the carpeting plants if you are not up for the challenge.