I like sumps, though Iím no expert. I have a 40g breeder with a 20 long as a sump. Hereís a website that can help calculate sizing: https://reefs.com/calculators/sump-t...me-calculator/
As for a pump, in freshwater the rule of thumb is you want to have a water turnover rate of roughly 8-10 times per hour, so youíd want to size your pump for that. You can get a decent Chinese knockoff DC
variable flow pump that will let you control the flow more precisely for not much more than a regular pump. Be sure to actually size it taking in to account height head losses, and fittings head losses too if you want to be crazy. I canít really remember by fluids formulas from college to help you more specifically but hopefully thatíll give you a place to start looking
As for filtration (I assume youíre talking filter media and the like?) it kind of depends on what youíre planning on stocking. Itís good to have some level of floss or a filter sock or something first to get the big chunks. For basic bio media, lots of diy sumps have those cheap plastic pot scrubbies. I have a bunch of landscaping volcano rock in mine, with some ďrealĒ Seachem matrix and purigen. Generally there will be different chambers in a sump for you to put different stuff in.
I can try to help more if youíve got more questions. Though a lot of what I know of just from seeing what other people have done, so do a search and start finding things you like. Thatís the best part of a diy sump, in my opinion, you can keep tinkering with it for as long as you like